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/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>()