Fix device sync: folder status badges, remove-deletes-local, multi-select, header summary

- Folders now show the synced status badge themselves, not just their
  nested files - device-sync state never tracked folders individually, so
  syncStatusFor derives a folder's badge from whether its path falls
  within sync scope instead.
- Turning sync off for a path now deletes its local mirror and clears its
  native sync-state entries (SyncEngine.removeLocalSync), instead of just
  stopping future updates and leaving the downloaded copy behind.
- "Sync to device" in Files' selection toolbar now works over the whole
  selection at once instead of being gated to a single selected item.
- The sync header's expanded panel shows real folder/item counts instead
  of a static "Synced" label that stayed accurate-sounding even before
  anything had actually synced.
- Confirmation SnackBars on starting/stopping sync from both the
  selection toolbar and Settings' Device Sync card.

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

lib/services/sync_service.dart

@@ -149,6 +149,23 @@ class SyncService {
     });
   }
 
+  /// Deletes [path]'s local mirror and its recorded sync state - called
+  /// when the user turns sync off for a path, so the on-device copy
+  /// actually goes away instead of just stopping future updates. See
+  /// `SyncEngine.removeLocalSync`'s doc comment for why the state also has
+  /// to be cleared, not just the files.
+  static Future<void> removeLocalSync(
+    SessionController session,
+    String path,
+  ) async {
+    final accountId = session.activeAccountId;
+    if (accountId == null) return;
+    await _channel.invokeMethod('removeLocalSync', {
+      'accountId': accountId,
+      'path': path,
+    });
+  }
+
   /// The deterministic local mirror path for [remoteItemPath] under
   /// account [accountId] - mirrors `SyncEngine.kt#syncRoot`'s
   /// `<externalFilesDir>/sync/<accountId>/...` layout exactly, so this