solitaire_data/src/lib.rs

@@ -133,6 +133,12 @@ pub use challenge::{CHALLENGE_SEEDS, challenge_count, challenge_seed_for};
 pub mod difficulty_seeds;
 pub use difficulty_seeds::{DifficultySeeds, seeds_for};
 
+pub mod transfer;
+pub use transfer::{
+    BundleError, DATA_BUNDLE_SCHEMA_VERSION, DataBundle, data_bundle_path, load_bundle_from,
+    save_bundle_to,
+};
+
 pub mod settings;
 pub use settings::{
     AnimSpeed, REPLAY_MOVE_INTERVAL_STEP_SECS, SOLVER_DEAL_RETRY_CAP, Settings, SyncBackend,

solitaire_data/src/transfer.rs

@@ -0,0 +1,137 @@
+//! Local data export/import (Phase M — sync transparency).
+//!
+//! Bundles every player-owned JSON save (settings, stats, achievements,
+//! progress) into one versioned backup file so a player can move their
+//! data between devices without an account, or keep a manual backup.
+//!
+//! The bundle deliberately excludes the in-progress game and the replay
+//! history: the former is transient, the latter can be large and is
+//! shareable through the replay upload path instead.
+
+use std::fs;
+use std::io;
+use std::path::{Path, PathBuf};
+
+use chrono::{DateTime, Utc};
+use serde::{Deserialize, Serialize};
+
+use crate::settings::Settings;
+use solitaire_sync::{AchievementRecord, PlayerProgress, StatsSnapshot};
+
+/// Bundle format version. Bump on any breaking shape change; the loader
+/// rejects newer versions rather than misreading them.
+pub const DATA_BUNDLE_SCHEMA_VERSION: u32 = 1;
+
+/// File name of the default backup location (next to the other saves).
+pub const DATA_BUNDLE_FILE_NAME: &str = "ferrous-solitaire-backup.json";
+
+/// One-file backup of every player-owned JSON save.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DataBundle {
+    /// Format version — see [`DATA_BUNDLE_SCHEMA_VERSION`].
+    pub schema_version: u32,
+    /// When the bundle was written.
+    pub exported_at: DateTime<Utc>,
+    pub settings: Settings,
+    pub stats: StatsSnapshot,
+    pub achievements: Vec<AchievementRecord>,
+    pub progress: PlayerProgress,
+}
+
+/// Why a bundle could not be loaded.
+#[derive(Debug, thiserror::Error)]
+pub enum BundleError {
+    #[error("could not read backup file: {0}")]
+    Io(#[from] io::Error),
+    #[error("backup file is not valid JSON: {0}")]
+    Parse(#[from] serde_json::Error),
+    #[error("backup was written by a newer app version (schema {found} > {supported})")]
+    UnsupportedVersion { found: u32, supported: u32 },
+}
+
+/// Default backup path: `<data dir>/<app dir>/ferrous-solitaire-backup.json`,
+/// or `None` when no data directory is available on this platform.
+pub fn data_bundle_path() -> Option<PathBuf> {
+    crate::data_dir().map(|d| d.join(crate::APP_DIR_NAME).join(DATA_BUNDLE_FILE_NAME))
+}
+
+/// Write `bundle` to `path` atomically (`.tmp` → rename), creating parent
+/// directories as needed. Mirrors the other save-file writers.
+pub fn save_bundle_to(path: &Path, bundle: &DataBundle) -> io::Result<()> {
+    if let Some(parent) = path.parent() {
+        fs::create_dir_all(parent)?;
+    }
+    let json = serde_json::to_string_pretty(bundle).map_err(io::Error::other)?;
+    let tmp = path.with_extension("json.tmp");
+    fs::write(&tmp, json.as_bytes())?;
+    fs::rename(&tmp, path)?;
+    Ok(())
+}
+
+/// Read and validate a bundle from `path`. Unlike the per-file loaders this
+/// does NOT default-on-error — an import silently replacing player data
+/// with defaults would be destructive, so every failure is surfaced.
+pub fn load_bundle_from(path: &Path) -> Result<DataBundle, BundleError> {
+    let data = fs::read(path)?;
+    let bundle: DataBundle = serde_json::from_slice(&data)?;
+    if bundle.schema_version > DATA_BUNDLE_SCHEMA_VERSION {
+        return Err(BundleError::UnsupportedVersion {
+            found: bundle.schema_version,
+            supported: DATA_BUNDLE_SCHEMA_VERSION,
+        });
+    }
+    Ok(bundle)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn sample_bundle() -> DataBundle {
+        DataBundle {
+            schema_version: DATA_BUNDLE_SCHEMA_VERSION,
+            exported_at: Utc::now(),
+            settings: Settings::default(),
+            stats: StatsSnapshot {
+                games_played: 7,
+                ..Default::default()
+            },
+            achievements: vec![],
+            progress: PlayerProgress::default(),
+        }
+    }
+
+    #[test]
+    fn round_trips_through_disk() {
+        let dir = std::env::temp_dir().join(format!("bundle_test_{}", std::process::id()));
+        let path = dir.join(DATA_BUNDLE_FILE_NAME);
+        let bundle = sample_bundle();
+        save_bundle_to(&path, &bundle).expect("save should succeed");
+        let loaded = load_bundle_from(&path).expect("load should succeed");
+        assert_eq!(loaded.schema_version, DATA_BUNDLE_SCHEMA_VERSION);
+        assert_eq!(loaded.stats.games_played, 7);
+        let _ = fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn rejects_newer_schema() {
+        let dir = std::env::temp_dir().join(format!("bundle_test_v_{}", std::process::id()));
+        let path = dir.join(DATA_BUNDLE_FILE_NAME);
+        let mut bundle = sample_bundle();
+        bundle.schema_version = DATA_BUNDLE_SCHEMA_VERSION + 1;
+        save_bundle_to(&path, &bundle).expect("save should succeed");
+        assert!(matches!(
+            load_bundle_from(&path),
+            Err(BundleError::UnsupportedVersion { .. })
+        ));
+        let _ = fs::remove_dir_all(&dir);
+    }
+
+    #[test]
+    fn missing_file_is_an_io_error() {
+        assert!(matches!(
+            load_bundle_from(Path::new("/nonexistent/backup.json")),
+            Err(BundleError::Io(_))
+        ));
+    }
+}

solitaire_engine/src/events.rs

@@ -151,6 +151,18 @@ pub struct SyncLogoutRequestEvent;
 #[derive(Message, Debug, Clone, Copy, Default)]
 pub struct DeleteAccountRequestEvent;
 
+/// Request to export every JSON save into one local backup bundle
+/// (Phase M — sync transparency). Fired by the Account tab's "Export
+/// data" button; handled in the `Last` schedule by `settings_plugin`.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct DataExportRequestEvent;
+
+/// Request to restore every JSON save from the local backup bundle
+/// (Phase M — sync transparency). Fired by the Account tab's "Import
+/// data" button; handled in the `Last` schedule by `settings_plugin`.
+#[derive(Message, Debug, Clone, Copy, Default)]
+pub struct DataImportRequestEvent;
+
 /// Request to toggle the pause overlay. Fired by the HUD "Pause" button so
 /// the same toggle path runs whether the player presses `Esc` or clicks.
 /// Consumed by `pause_plugin::toggle_pause`, which honours the same drag /

solitaire_engine/src/resources.rs

@@ -102,6 +102,25 @@ pub enum SyncStatus {
 #[derive(Resource, Debug, Clone, Default)]
 pub struct SyncStatusResource(pub SyncStatus);
 
+/// Outcome of the most recent successful sync merge (Phase M — sync
+/// transparency). `None` until the first pull of the session resolves.
+///
+/// The Account tab renders the conflict details; the sync poller emits a
+/// one-line toast summary when `conflicts` is non-empty.
+#[derive(Resource, Debug, Clone, Default)]
+pub struct SyncConflictLog(pub Option<SyncMergeSummary>);
+
+/// Timestamp + per-field conflict list captured from
+/// [`solitaire_sync::merge`] after a pull resolves.
+#[derive(Debug, Clone)]
+pub struct SyncMergeSummary {
+    /// When the merge resolved.
+    pub at: DateTime<Utc>,
+    /// Fields that could not be merged deterministically (best-effort
+    /// resolution already applied — see `solitaire_sync::merge`).
+    pub conflicts: Vec<solitaire_sync::ConflictReport>,
+}
+
 /// Tracks which hint the player is currently cycling through.
 ///
 /// Incremented on each H press so repeated presses reveal different moves.

solitaire_engine/src/settings_plugin/data_transfer.rs

@@ -0,0 +1,139 @@
+//! Phase M (sync transparency): local backup export/import.
+//!
+//! Runs in the `Last` schedule — the export snapshot is taken after the
+//! whole frame settles, and the import's resource rewrites become visible
+//! to every Update system on the next frame, keeping this rare bulk
+//! operation off the annotated Update spine entirely.
+
+use bevy::prelude::*;
+use chrono::Utc;
+use solitaire_data::{DataBundle, data_bundle_path, load_bundle_from, save_bundle_to};
+
+use crate::achievement_plugin::{AchievementsResource, AchievementsStoragePath};
+use crate::events::{
+    DataExportRequestEvent, DataImportRequestEvent, InfoToastEvent, WarningToastEvent,
+};
+use crate::progress_plugin::{ProgressResource, ProgressStoragePath};
+use crate::stats_plugin::{StatsResource, StatsStoragePath};
+
+use super::{SettingsChangedEvent, SettingsResource, SettingsStoragePath};
+
+/// Drains the export/import request streams and performs the file IO.
+///
+/// The bundle is small (a few KB of JSON), so the write happens inline
+/// rather than on the async pool — one synchronous write per explicit
+/// button press, never per frame.
+#[allow(clippy::too_many_arguments)]
+pub(super) fn handle_data_transfer(
+    mut exports: MessageReader<DataExportRequestEvent>,
+    mut imports: MessageReader<DataImportRequestEvent>,
+    mut settings: ResMut<SettingsResource>,
+    settings_path: Res<SettingsStoragePath>,
+    stats: Option<ResMut<StatsResource>>,
+    stats_path: Option<Res<StatsStoragePath>>,
+    achievements: Option<ResMut<AchievementsResource>>,
+    achievements_path: Option<Res<AchievementsStoragePath>>,
+    progress: Option<ResMut<ProgressResource>>,
+    progress_path: Option<Res<ProgressStoragePath>>,
+    mut settings_changed: MessageWriter<SettingsChangedEvent>,
+    mut info_toast: MessageWriter<InfoToastEvent>,
+    mut warning_toast: MessageWriter<WarningToastEvent>,
+) {
+    let export_requested = exports.read().next().is_some();
+    let import_requested = imports.read().next().is_some();
+    if !export_requested && !import_requested {
+        return;
+    }
+    // The bundle lives next to settings.json (same app data dir in
+    // production); deriving it from the injected settings path keeps
+    // tests off the real platform directory.
+    let path = settings_path
+        .0
+        .as_ref()
+        .map(|p| p.with_file_name(solitaire_data::transfer::DATA_BUNDLE_FILE_NAME))
+        .or_else(data_bundle_path);
+    let Some(path) = path else {
+        warning_toast.write(WarningToastEvent(
+            "No data directory available on this platform".to_string(),
+        ));
+        return;
+    };
+
+    // An import in the same frame as an export would race on the file;
+    // real UI can't produce both, but be deterministic anyway: export
+    // first, then import reads what was just written (a no-op restore).
+    if export_requested {
+        let bundle = DataBundle {
+            schema_version: solitaire_data::DATA_BUNDLE_SCHEMA_VERSION,
+            exported_at: Utc::now(),
+            settings: settings.0.clone(),
+            stats: stats.as_ref().map(|s| s.0.clone()).unwrap_or_default(),
+            achievements: achievements
+                .as_ref()
+                .map(|a| a.0.clone())
+                .unwrap_or_default(),
+            progress: progress.as_ref().map(|p| p.0.clone()).unwrap_or_default(),
+        };
+        match save_bundle_to(&path, &bundle) {
+            Ok(()) => {
+                info_toast.write(InfoToastEvent(format!(
+                    "Data exported to {}",
+                    path.display()
+                )));
+            }
+            Err(e) => {
+                warn!("data export failed: {e}");
+                warning_toast.write(WarningToastEvent(format!("Export failed: {e}")));
+            }
+        }
+    }
+
+    if import_requested {
+        match load_bundle_from(&path) {
+            Ok(bundle) => {
+                settings.0 = bundle.settings;
+                if let Some(target) = &settings_path.0
+                    && let Err(e) = solitaire_data::save_settings_to(target, &settings.0)
+                {
+                    warn!("import: failed to persist settings: {e}");
+                }
+                // Settings appliers (theme, audio, layout) react to the
+                // change event exactly as if edited in the panel.
+                settings_changed.write(SettingsChangedEvent(settings.0.clone()));
+
+                if let Some(mut s) = stats {
+                    s.0 = bundle.stats;
+                    if let Some(target) = stats_path.as_ref().and_then(|p| p.0.as_ref())
+                        && let Err(e) = solitaire_data::save_stats_to(target, &s.0)
+                    {
+                        warn!("import: failed to persist stats: {e}");
+                    }
+                }
+                if let Some(mut a) = achievements {
+                    a.0 = bundle.achievements;
+                    if let Some(target) = achievements_path.as_ref().and_then(|p| p.0.as_ref())
+                        && let Err(e) = solitaire_data::save_achievements_to(target, &a.0)
+                    {
+                        warn!("import: failed to persist achievements: {e}");
+                    }
+                }
+                if let Some(mut p) = progress {
+                    p.0 = bundle.progress;
+                    if let Some(target) = progress_path.as_ref().and_then(|pp| pp.0.as_ref())
+                        && let Err(e) = solitaire_data::save_progress_to(target, &p.0)
+                    {
+                        warn!("import: failed to persist progress: {e}");
+                    }
+                }
+                info_toast.write(InfoToastEvent(format!(
+                    "Backup from {} restored",
+                    bundle.exported_at.format("%Y-%m-%d %H:%M UTC")
+                )));
+            }
+            Err(e) => {
+                warn!("data import failed: {e}");
+                warning_toast.write(WarningToastEvent(format!("Import failed: {e}")));
+            }
+        }
+    }
+}

solitaire_engine/src/settings_plugin/input.rs

@@ -433,6 +433,9 @@ pub(super) fn handle_settings_buttons(
             SettingsButton::OpenThemeStore => {
                 // Handled by `handle_sync_buttons`.
             }
+            SettingsButton::ExportData | SettingsButton::ImportData => {
+                // Handled by `handle_sync_buttons`.
+            }
             SettingsButton::Done => {
                 screen.0 = false;
             }
@@ -460,14 +463,18 @@ pub(super) fn handle_tab_buttons(
 }
 
 /// Handles sync-related settings buttons: Sync Now, Connect, Disconnect,
-/// and Delete Account. Split from `handle_settings_buttons` to stay within
-/// Bevy's 16-parameter system limit.
+/// Delete Account, and the Phase M Export/Import data pair. Split from
+/// `handle_settings_buttons` to stay within Bevy's 16-parameter system
+/// limit.
+#[allow(clippy::too_many_arguments)]
 pub(super) fn handle_sync_buttons(
     interaction_query: Query<(&Interaction, &SettingsButton), Changed<Interaction>>,
     mut manual_sync: MessageWriter<ManualSyncRequestEvent>,
     mut configure_sync: MessageWriter<SyncConfigureRequestEvent>,
     mut logout_sync: MessageWriter<SyncLogoutRequestEvent>,
     mut delete_account: MessageWriter<DeleteAccountRequestEvent>,
+    mut export_data: MessageWriter<crate::events::DataExportRequestEvent>,
+    mut import_data: MessageWriter<crate::events::DataImportRequestEvent>,
     #[cfg(not(target_arch = "wasm32"))] mut open_theme_store: MessageWriter<
         crate::events::ThemeStoreOpenRequestEvent,
     >,
@@ -500,6 +507,12 @@ pub(super) fn handle_sync_buttons(
             SettingsButton::DeleteAccount => {
                 delete_account.write(DeleteAccountRequestEvent);
             }
+            SettingsButton::ExportData => {
+                export_data.write(crate::events::DataExportRequestEvent);
+            }
+            SettingsButton::ImportData => {
+                import_data.write(crate::events::DataImportRequestEvent);
+            }
             _ => {}
         }
     }

solitaire_engine/src/settings_plugin/mod.rs

@@ -25,6 +25,7 @@ use crate::events::{
 use crate::resources::SettingsScrollPos;
 use crate::theme::ThemeThumbnailPair;
 
+mod data_transfer;
 mod input;
 mod ui;
 mod updates;
@@ -124,6 +125,12 @@ struct ThemeText;
 #[derive(Component, Debug)]
 struct SyncStatusText;
 
+/// Marks the headline `Text` node of the last-merge summary in the
+/// Account tab (Phase M — sync transparency). Tests query it to assert
+/// the summary rendered.
+#[derive(Component, Debug)]
+pub(super) struct SyncMergeSummaryText;
+
 /// Marks the `Text` node showing the active card-back index.
 #[derive(Component, Debug)]
 struct CardBackText;
@@ -334,6 +341,10 @@ enum SettingsButton {
     DisconnectSync,
     /// Open the account-deletion confirmation modal.
     DeleteAccount,
+    /// Write every JSON save into one backup bundle (Phase M).
+    ExportData,
+    /// Restore every JSON save from the backup bundle (Phase M).
+    ImportData,
     Done,
     /// Select a specific card-back by index from the picker row.
     SelectCardBack(usize),
@@ -399,6 +410,8 @@ impl SettingsButton {
             SettingsButton::ConnectSync => 91,
             SettingsButton::DisconnectSync => 92,
             SettingsButton::DeleteAccount => 93,
+            SettingsButton::ExportData => 94,
+            SettingsButton::ImportData => 95,
             // Done is tagged by `attach_focusable_to_modal_buttons` and
             // never reaches `attach_focusable_to_settings_buttons`; the
             // value here is only a fallback for completeness.
@@ -452,6 +465,15 @@ impl Plugin for SettingsPlugin {
             .add_message::<SyncConfigureRequestEvent>()
             .add_message::<SyncLogoutRequestEvent>()
             .add_message::<DeleteAccountRequestEvent>()
+            .add_message::<crate::events::DataExportRequestEvent>()
+            .add_message::<crate::events::DataImportRequestEvent>()
+            // Idempotent — the sync plugin also registers it; needed here
+            // so the transfer handler's failure toasts work headless.
+            .add_message::<crate::events::WarningToastEvent>()
+            // Bulk backup/restore lives in Last: the export snapshot is
+            // taken post-frame and an import's rewrites surface next
+            // frame — keeps the rare path off the annotated Update spine.
+            .add_systems(Last, data_transfer::handle_data_transfer)
             .add_message::<ToggleSettingsRequestEvent>()
             .add_message::<crate::events::ThemeStoreOpenRequestEvent>()
             .add_message::<InfoToastEvent>()

solitaire_engine/src/settings_plugin/tests.rs

@@ -793,3 +793,93 @@ fn ui_scale_out_of_range_sanitizes_on_load() {
     .sanitized();
     assert_eq!(tiny.ui_scale, UI_SCALE_MIN);
 }
+
+// ---------------------------------------------------------------------------
+// Phase M: local data export / import
+// ---------------------------------------------------------------------------
+
+/// App with a real (temp-dir) settings path so the data-transfer handler
+/// derives its bundle path inside the temp dir instead of the platform
+/// data directory.
+fn transfer_app(dir: &std::path::Path) -> App {
+    let mut app = App::new();
+    app.add_plugins(MinimalPlugins).add_plugins(SettingsPlugin {
+        storage_path: Some(dir.join("settings.json")),
+        ui_enabled: false,
+    });
+    app.init_resource::<ButtonInput<KeyCode>>();
+    app.update();
+    app
+}
+
+#[test]
+fn export_then_import_round_trips_settings() {
+    let dir = std::env::temp_dir().join(format!("settings_transfer_{}", std::process::id()));
+    std::fs::create_dir_all(&dir).expect("temp dir");
+    let mut app = transfer_app(&dir);
+
+    app.world_mut()
+        .resource_mut::<SettingsResource>()
+        .0
+        .sfx_volume = 0.25;
+    app.world_mut()
+        .write_message(crate::events::DataExportRequestEvent);
+    app.update();
+    assert!(
+        dir.join(solitaire_data::transfer::DATA_BUNDLE_FILE_NAME)
+            .exists(),
+        "export should write the bundle next to settings.json"
+    );
+
+    // Clobber the live value, then restore from the bundle.
+    app.world_mut()
+        .resource_mut::<SettingsResource>()
+        .0
+        .sfx_volume = 1.0;
+    app.world_mut()
+        .write_message(crate::events::DataImportRequestEvent);
+    app.update();
+    // Import handling runs in Last of the same frame; the rewrite is
+    // visible immediately after update() returns.
+    let restored = app.world().resource::<SettingsResource>().0.sfx_volume;
+    assert!(
+        (restored - 0.25).abs() < f32::EPSILON,
+        "import should restore the exported value, got {restored}"
+    );
+
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
+#[test]
+fn import_without_backup_warns_instead_of_defaulting() {
+    let dir =
+        std::env::temp_dir().join(format!("settings_transfer_missing_{}", std::process::id()));
+    std::fs::create_dir_all(&dir).expect("temp dir");
+    let mut app = transfer_app(&dir);
+
+    app.world_mut()
+        .resource_mut::<SettingsResource>()
+        .0
+        .sfx_volume = 0.25;
+    app.world_mut()
+        .write_message(crate::events::DataImportRequestEvent);
+    app.update();
+
+    // Settings untouched (no silent default-restore)…
+    let v = app.world().resource::<SettingsResource>().0.sfx_volume;
+    assert!(
+        (v - 0.25).abs() < f32::EPSILON,
+        "failed import must not touch settings"
+    );
+    // …and the failure is surfaced as a warning toast.
+    let msgs = app
+        .world()
+        .resource::<Messages<crate::events::WarningToastEvent>>();
+    let mut cursor = msgs.get_cursor();
+    assert!(
+        cursor.read(msgs).any(|w| w.0.contains("Import failed")),
+        "missing backup should emit a warning toast"
+    );
+
+    let _ = std::fs::remove_dir_all(&dir);
+}

solitaire_engine/src/settings_plugin/ui.rs

@@ -46,6 +46,7 @@ pub(super) fn spawn_settings_panel(
     scroll_offset: f32,
     font_res: Option<&FontResource>,
     theme_overrides_back: bool,
+    conflict_log: Option<&crate::resources::SyncConflictLog>,
     active_tab: SettingsTab,
 ) {
     spawn_modal(commands, SettingsPanel, Z_MODAL_PANEL, |card| {
@@ -100,7 +101,9 @@ pub(super) fn spawn_settings_panel(
                 font_res,
             ),
             SettingsTab::Accessibility => spawn_accessibility_tab(body, settings, font_res),
-            SettingsTab::Account => spawn_account_tab(body, settings, sync_status, font_res),
+            SettingsTab::Account => {
+                spawn_account_tab(body, settings, sync_status, conflict_log, font_res)
+            }
         });
 
         // Done is the only action — primary so the player always knows
@@ -353,6 +356,7 @@ fn spawn_account_tab(
     body: &mut ChildSpawnerCommands,
     settings: &Settings,
     sync_status: &str,
+    conflict_log: Option<&crate::resources::SyncConflictLog>,
     font_res: Option<&FontResource>,
 ) {
     // --- Privacy (only shown when a Matomo URL is configured) ---
@@ -372,6 +376,144 @@ fn spawn_account_tab(
     // --- Sync ---
     section_label(body, "Sync", font_res);
     sync_row(body, sync_status, &settings.sync_backend, font_res);
+    spawn_sync_merge_summary(body, conflict_log, font_res);
+
+    // --- Local data (Phase M) --- backup/restore of the JSON saves;
+    // filesystem-backed, so not offered on the wasm build.
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        section_label(body, "Local data", font_res);
+        local_data_row(body, font_res);
+    }
+}
+
+/// `Export data` / `Import data` pill buttons plus a caption naming the
+/// backup file (Phase M). Mirrors the sync row's pill-button styling.
+#[cfg(not(target_arch = "wasm32"))]
+fn local_data_row(parent: &mut ChildSpawnerCommands, font_res: Option<&FontResource>) {
+    let button_font = TextFont {
+        font: font_res.map(|f| f.0.clone()).unwrap_or_default(),
+        font_size: TYPE_CAPTION,
+        ..default()
+    };
+    let caption_font = button_font.clone();
+
+    let small_button = |row: &mut ChildSpawnerCommands,
+                        marker: SettingsButton,
+                        label: &str,
+                        tooltip: String,
+                        font: TextFont| {
+        row.spawn((
+            marker,
+            Button,
+            Tooltip::new(tooltip),
+            Node {
+                padding: UiRect::axes(VAL_SPACE_3, VAL_SPACE_2),
+                justify_content: JustifyContent::Center,
+                border: UiRect::all(Val::Px(1.0)),
+                border_radius: BorderRadius::all(Val::Px(RADIUS_SM)),
+                ..default()
+            },
+            BackgroundColor(BG_ELEVATED_HI),
+            BorderColor::all(BORDER_SUBTLE),
+            HighContrastBorder::with_default(BORDER_SUBTLE),
+        ))
+        .with_children(|b| {
+            b.spawn((Text::new(label.to_string()), font, TextColor(TEXT_PRIMARY)));
+        });
+    };
+
+    parent
+        .spawn(Node {
+            flex_direction: FlexDirection::Column,
+            row_gap: VAL_SPACE_2,
+            ..default()
+        })
+        .with_children(|col| {
+            col.spawn(Node {
+                flex_direction: FlexDirection::Row,
+                align_items: AlignItems::Center,
+                column_gap: VAL_SPACE_3,
+                flex_wrap: FlexWrap::Wrap,
+                row_gap: VAL_SPACE_2,
+                ..default()
+            })
+            .with_children(|row| {
+                small_button(
+                    row,
+                    SettingsButton::ExportData,
+                    "Export data",
+                    "Write settings, stats, achievements and progress to one backup file"
+                        .to_string(),
+                    button_font.clone(),
+                );
+                small_button(
+                    row,
+                    SettingsButton::ImportData,
+                    "Import data",
+                    "Restore all saves from the backup file (replaces current data)".to_string(),
+                    button_font.clone(),
+                );
+            });
+            col.spawn((
+                Text::new(format!(
+                    "Backup file: {}",
+                    solitaire_data::data_bundle_path()
+                        .map_or_else(|| "unavailable".to_string(), |p| p.display().to_string())
+                )),
+                caption_font,
+                TextColor(TEXT_SECONDARY),
+            ));
+        });
+}
+
+/// Phase M (sync transparency): renders what the most recent sync merge
+/// did — a clean-merge caption, or one line per conflicting field with
+/// both values, so the player can see exactly which numbers were
+/// reconciled and how.
+fn spawn_sync_merge_summary(
+    body: &mut ChildSpawnerCommands,
+    conflict_log: Option<&crate::resources::SyncConflictLog>,
+    font_res: Option<&FontResource>,
+) {
+    let Some(summary) = conflict_log.and_then(|log| log.0.as_ref()) else {
+        return;
+    };
+    let caption_font = TextFont {
+        font: font_res.map(|f| f.0.clone()).unwrap_or_default(),
+        font_size: TYPE_CAPTION,
+        ..default()
+    };
+    let when = summary.at.format("%H:%M UTC");
+    if summary.conflicts.is_empty() {
+        body.spawn((
+            SyncMergeSummaryText,
+            Text::new(format!("Last merge at {when}: no conflicts")),
+            caption_font,
+            TextColor(TEXT_SECONDARY),
+        ));
+        return;
+    }
+    let n = summary.conflicts.len();
+    let plural = if n == 1 { "conflict" } else { "conflicts" };
+    body.spawn((
+        SyncMergeSummaryText,
+        Text::new(format!(
+            "Last merge at {when}: {n} {plural} — kept newer values"
+        )),
+        caption_font.clone(),
+        TextColor(TEXT_SECONDARY),
+    ));
+    for c in &summary.conflicts {
+        body.spawn((
+            Text::new(format!(
+                "  {} — this device: {} / server: {}",
+                c.field, c.local_value, c.remote_value
+            )),
+            caption_font.clone(),
+            TextColor(TEXT_SECONDARY),
+        ));
+    }
 }
 
 /// Section divider — small lavender label inside the scrollable body.

solitaire_engine/src/settings_plugin/updates.rs

@@ -32,6 +32,7 @@ pub(super) fn sync_settings_panel_visibility(
     theme_registry: Option<Res<crate::theme::ThemeRegistry>>,
     theme_thumbs: Option<Res<ThemeThumbnailCache>>,
     card_images: Option<Res<crate::card_plugin::CardImageSet>>,
+    conflict_log: Option<Res<crate::resources::SyncConflictLog>>,
 ) {
     if !screen.is_changed() {
         return;
@@ -47,6 +48,7 @@ pub(super) fn sync_settings_panel_visibility(
                 theme_registry.as_deref(),
                 theme_thumbs.as_deref(),
                 card_images.as_deref(),
+                conflict_log.as_deref(),
                 scroll_pos.0,
                 active_tab.0,
             );
@@ -76,6 +78,7 @@ fn build_panel(
     theme_registry: Option<&crate::theme::ThemeRegistry>,
     theme_thumbs: Option<&ThemeThumbnailCache>,
     card_images: Option<&crate::card_plugin::CardImageSet>,
+    conflict_log: Option<&crate::resources::SyncConflictLog>,
     scroll_offset: f32,
     active_tab: SettingsTab,
 ) {
@@ -125,6 +128,7 @@ fn build_panel(
         scroll_offset,
         font_res,
         theme_overrides_back,
+        conflict_log,
         active_tab,
     );
 }
@@ -145,6 +149,7 @@ pub(super) fn rebuild_panel_on_tab_change(
     theme_registry: Option<Res<crate::theme::ThemeRegistry>>,
     theme_thumbs: Option<Res<ThemeThumbnailCache>>,
     card_images: Option<Res<crate::card_plugin::CardImageSet>>,
+    conflict_log: Option<Res<crate::resources::SyncConflictLog>>,
 ) {
     if !active_tab.is_changed() || active_tab.is_added() {
         return;
@@ -167,6 +172,7 @@ pub(super) fn rebuild_panel_on_tab_change(
         theme_registry.as_deref(),
         theme_thumbs.as_deref(),
         card_images.as_deref(),
+        conflict_log.as_deref(),
         0.0,
         active_tab.0,
     );

solitaire_engine/src/sync_plugin.rs

@@ -26,11 +26,15 @@ use solitaire_sync::{SyncPayload, merge};
 
 use crate::achievement_plugin::{AchievementsResource, AchievementsStoragePath};
 use crate::events::{
-    GameWonEvent, ManualSyncRequestEvent, SyncConfigureRequestEvent, WarningToastEvent,
+    GameWonEvent, InfoToastEvent, ManualSyncRequestEvent, SyncConfigureRequestEvent,
+    WarningToastEvent,
 };
 use crate::game_plugin::RecordingReplay;
 use crate::progress_plugin::{ProgressResource, ProgressStoragePath};
-use crate::resources::{GameStateResource, SyncStatus, SyncStatusResource, TokioRuntimeResource};
+use crate::resources::{
+    GameStateResource, SyncConflictLog, SyncMergeSummary, SyncStatus, SyncStatusResource,
+    TokioRuntimeResource,
+};
 use crate::stats_plugin::{
     LatestReplayPath, ReplayHistoryResource, StatsResource, StatsStoragePath,
 };
@@ -106,9 +110,13 @@ impl Plugin for SyncPlugin {
             .init_resource::<PullTaskResult>()
             .init_resource::<PullTask>()
             .init_resource::<PendingReplayUpload>()
+            .init_resource::<SyncConflictLog>()
             .add_message::<ManualSyncRequestEvent>()
             .add_message::<SyncConfigureRequestEvent>()
-            .add_message::<WarningToastEvent>();
+            .add_message::<WarningToastEvent>()
+            // Idempotent — GamePlugin also registers it; re-register so the
+            // conflict-summary toast works under MinimalPlugins tests.
+            .add_message::<InfoToastEvent>();
 
         // Build the shared Tokio runtime; disable all network sync if the OS
         // refuses to create threads (resource-limited environments, sandboxed
@@ -198,6 +206,8 @@ fn poll_pull_result(
     progress_path: Res<ProgressStoragePath>,
     mut configure_sync: MessageWriter<SyncConfigureRequestEvent>,
     mut warning_toast: MessageWriter<WarningToastEvent>,
+    mut conflict_log: ResMut<SyncConflictLog>,
+    mut info_toast: MessageWriter<InfoToastEvent>,
 ) {
     let Some(task) = task_res.0.as_mut() else {
         return;
@@ -210,7 +220,21 @@ fn poll_pull_result(
     match result {
         Ok(remote) => {
             let local = build_payload(&stats.0, &achievements.0, &progress.0);
-            let (merged, _conflicts) = merge(&local, &remote);
+            let (merged, conflicts) = merge(&local, &remote);
+
+            // Phase M (sync transparency): keep the conflict details for
+            // the Account tab and summarise non-clean merges in a toast.
+            if !conflicts.is_empty() {
+                let n = conflicts.len();
+                let plural = if n == 1 { "conflict" } else { "conflicts" };
+                info_toast.write(InfoToastEvent(format!(
+                    "Synced — {n} {plural}, kept newer values"
+                )));
+            }
+            conflict_log.0 = Some(SyncMergeSummary {
+                at: Utc::now(),
+                conflicts,
+            });
 
             // Persist merged state atomically.
             if let Some(p) = &stats_path.0