feat(flutter): drift-first Library tabs + systemPlaylistsStatus + tab pre-warm

Four-part change to push more surfaces onto the drift cache and
eliminate cold-tab-visit latency on the Library screen.

* **Library Artists tab** — _libraryArtistsProvider migrates from
  REST-paginated AsyncNotifier with infinite-scroll loadMore to a
  drift-first StreamProvider over cached_artists ordered by
  sortName. Sync already populates the full set; cacheFirst's
  fetchAndPopulate covers the fresh-install + sync-not-yet-done
  cold case via /api/artists?limit=1000. SWR refresh on every
  visit. GridView.builder lazily realizes only visible cells so
  loading the full list up front is fine for typical libraries.
  loadMore + NotificationListener gone.

* **Library Albums tab** — same migration, drift-first over
  cached_albums joined with cached_artists for the artistName
  field.

* **systemPlaylistsStatusProvider** — new CachedSystemPlaylistsStatus
  single-row table (schema 6 → 7, JSON blob like CachedHomeSnapshot)
  for the home Playlists row's "building / pending / failed"
  placeholder logic. Drift-first means the row paints with the
  prior status instantly instead of flickering through
  SystemPlaylistsStatus.empty() while the REST call resolves.

* **Library screen tab pre-warm** — ref.listen on all 5 tab
  providers in _LibraryScreenState.build subscribes them upfront
  so swiping between tabs feels instant rather than each tab
  paying its own cold-cache cost on first visit. cacheFirst
  handles dedupe of concurrent fetchAndPopulate triggers.

Test mock updated for the StreamProvider type change on
systemPlaylistsStatusProvider.

flutter_client/test/library/home_screen_test.dart

@@ -28,7 +28,7 @@ void main() {
           (ref) => Stream.value(PlaylistsList.empty()),
         ),
         systemPlaylistsStatusProvider.overrideWith(
-          (ref) async => SystemPlaylistsStatus.empty(),
+          (ref) => Stream.value(SystemPlaylistsStatus.empty()),
         ),
       ],
       child: MaterialApp(theme: buildThemeData(), home: const HomeScreen()),
@@ -49,7 +49,7 @@ void main() {
           (ref) => Stream.value(PlaylistsList.empty()),
         ),
         systemPlaylistsStatusProvider.overrideWith(
-          (ref) async => SystemPlaylistsStatus.empty(),
+          (ref) => Stream.value(SystemPlaylistsStatus.empty()),
         ),
       ],
       child: MaterialApp(theme: buildThemeData(), home: const HomeScreen()),
@@ -91,7 +91,7 @@ void main() {
               const PlaylistsList(owned: [forYou], public: [])),
         ),
         systemPlaylistsStatusProvider.overrideWith(
-          (ref) async => SystemPlaylistsStatus.empty(),
+          (ref) => Stream.value(SystemPlaylistsStatus.empty()),
         ),
       ],
       child: MaterialApp(theme: buildThemeData(), home: const HomeScreen()),