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/views/account_view.dart

@@ -823,7 +823,15 @@ class _DeviceSyncCardState extends State<_DeviceSyncCard> {
                 trailing: IconButton(
                   icon: const Icon(Icons.close_rounded),
                   tooltip: 'Stop syncing',
-                  onPressed: () => sync.removeSyncedPath(folder),
+                  onPressed: () {
+                    sync.removeSyncedPath(folder);
+                    ScaffoldMessenger.of(context).showSnackBar(
+                      const SnackBar(
+                        content: Text('Removed from device sync'),
+                        behavior: SnackBarBehavior.floating,
+                      ),
+                    );
+                  },
                 ),
               ),
               const Divider(height: 1, indent: 16, endIndent: 16),