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/lib/playlists/playlists_provider.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:convert';
 
 import 'package:dio/dio.dart';
 import 'package:drift/drift.dart' as drift;
@@ -362,8 +363,41 @@ class _TrackRefRow {
   final int durationSec;
 }
 
+/// Drift-first per #357 pattern. Reads from cached_system_playlists_
+/// status (single-row JSON blob populated by /api/me/system-playlists-
+/// status). Home Playlists row reads this every render to choose
+/// between real cards and "building / pending / failed" placeholders
+/// — drift-first means the row paints with the prior status instantly
+/// rather than flickering through SystemPlaylistsStatus.empty() while
+/// the REST round-trip resolves. SWR refresh on every visit keeps it
+/// current.
 final systemPlaylistsStatusProvider =
-    FutureProvider<SystemPlaylistsStatus>((ref) async {
-  final dio = await ref.watch(dioProvider.future);
-  return MeApi(dio).systemPlaylistsStatus();
+    StreamProvider<SystemPlaylistsStatus>((ref) {
+  final db = ref.watch(appDbProvider);
+  return cacheFirst<CachedSystemPlaylistsStatusData, SystemPlaylistsStatus>(
+    driftStream: db.select(db.cachedSystemPlaylistsStatus).watch(),
+    fetchAndPopulate: () async {
+      final dio = await ref.read(dioProvider.future);
+      final fresh = await MeApi(dio).systemPlaylistsStatus();
+      await db.into(db.cachedSystemPlaylistsStatus).insertOnConflictUpdate(
+            CachedSystemPlaylistsStatusCompanion.insert(
+              json: jsonEncode({
+                'in_flight': fresh.inFlight,
+                'last_run_at': fresh.lastRunAt,
+                'last_error': fresh.lastError,
+              }),
+              updatedAt: drift.Value(DateTime.now()),
+            ),
+          );
+    },
+    toResult: (rows) => rows.isEmpty
+        ? SystemPlaylistsStatus.empty()
+        : SystemPlaylistsStatus.fromJson(
+            jsonDecode(rows.first.json) as Map<String, dynamic>),
+    isOnline: () async => (await ref
+        .read(connectivityProvider.future)
+        .timeout(const Duration(seconds: 3), onTimeout: () => true)),
+    alwaysRefresh: true,
+    tag: 'systemPlaylistsStatus',
+  );
 });