feat(engine): Phase M — sync transparency + local data backup

Closes out the 13-phase menu redesign. Three parts:

1. Post-sync merge summary: the pull poller now keeps the
   `ConflictReport`s the merge produces (previously discarded) in a new
   `SyncConflictLog` resource and toasts "Synced — N conflicts, kept
   newer values" when a merge wasn't clean. No wire changes — the
   server-side `SyncResponse.conflicts` field already existed.

2. Account tab detail: a per-field conflict list under the sync row
   ("win_streak_current — this device: 3 / server: 5") plus a
   clean-merge caption, so the last merge is always inspectable.

3. Local data export/import: new `solitaire_data::transfer` module —
   one versioned JSON bundle (settings, stats, achievements, progress)
   written atomically next to the other saves, with a version-gated
   loader that surfaces every failure instead of defaulting. Account
   tab grows an Export/Import button pair (desktop+Android only); the
   handler runs in `Last`, off the annotated Update spine. Import
   rewrites the live resources, persists via the existing save fns,
   and fires SettingsChangedEvent so appliers react normally.

Tests: bundle round-trip/version-gate/missing-file in solitaire_data;
ECS-level export→import round trip and failed-import-warns in
settings_plugin. Gate: workspace tests green, clippy -D warnings clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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.