feat(track): icon-based status badges, detect locally-cached tracks

Replace the labelled availability/metadata badges in track rows with
small icon+tooltip indicators (cloud/hard-drives/warning/etc, derived
from TrackAvailability and MetadataStatus).

Add a `connection` slice fed by a single status poller (Sidebar) so
other components can cheaply check backend reachability. TrackRow uses
this plus the offline audio cache to show "Local" instead of a stale
"On server" when the backend is down but the track is already cached.

src/components/layout/Sidebar.tsx

@@ -3,7 +3,7 @@ import { useTranslation } from 'react-i18next';
 import { Icon, type IconName } from '../common/Icon';
 import { useAppDispatch } from '../../hooks/useAppDispatch';
 import { usePermissions, type Permission } from '../../hooks/usePermissions';
-import { useConnectionStatus } from '../../hooks/useConnectionStatus';
+import { useConnectionStatusSync } from '../../hooks/useConnectionStatus';
 import { logout } from '../../store/slices/auth';
 import { useGetPlaylistsQuery } from '../../api/endpoints/playlists';
 import { getActiveInstance } from '../../config/instances';
@@ -41,7 +41,7 @@ export function Sidebar() {
   const dispatch = useAppDispatch();
   const navigate = useNavigate();
   const { user, isAdmin, hasPermission } = usePermissions();
-  const status = useConnectionStatus();
+  const status = useConnectionStatusSync();
   const { data: playlists } = useGetPlaylistsQuery();
   const instance = getActiveInstance();