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/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.