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/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);
+            }
             _ => {}
         }
     }