feat(flutter): home screen on per-item rendering (Slice C)

End-to-end pilot of the per-item architecture. Home now reads from
the new homeIndexProvider (drift-first over CachedHomeIndex with
/api/home/index discovery + SWR), then each tile is a small
ConsumerWidget watching its own albumTileProvider/artistTileProvider/
trackTileProvider. Tiles render a matched-dimension skeleton while
their entity is still hydrating, and swap in the real card once
drift emits the populated row.

Track hydration is wired up — /api/tracks/:id already existed so
the queue's case 'track' just calls api.getTrack(id) and persists.

The visible behavior:
* Cold visit: small /api/home/index round-trip (IDs only, ~10×
  smaller than /api/home), then sections appear shaped with
  skeleton tiles; each tile materializes as the hydration queue
  drains. No more "30s blank → everything pops in at once."
* Warm visit: drift index emits instantly, drift entity rows emit
  instantly, no network. Page paints fully in the first frame.
* Mid-state: scrolling through a partially-hydrated section sees
  real cards next to skeleton cards. Layout doesn't shift because
  skeletons match real-card dimensions exactly.

CachedHomeSnapshot (and the legacy homeProvider) stay in place but
unconsumed by Flutter — left in for now so revert is cheap if the
new path needs reworking. Cleanup follow-up in a later slice.

Old /api/home endpoint untouched, so the web client keeps working
unchanged.

flutter_client/lib/library/library_providers.dart

@@ -17,6 +17,7 @@ import '../cache/db.dart';
 import '../models/album.dart';
 import '../models/artist.dart';
 import '../models/home_data.dart';
+import '../models/home_index.dart';
 import '../models/track.dart';
 
 /// Shared authenticated dio. This is the ONLY place tokenResolver is wired
@@ -94,6 +95,95 @@ final homeProvider = StreamProvider<HomeData>((ref) {
   );
 });
 
+/// Drift-first per-item home index. Reads from cached_home_index
+/// (populated by /api/home/index discovery) and yields a HomeIndex
+/// the screen consumes to lay out section→tile slots. Each tile is
+/// rendered by a per-entity tile provider that hydrates itself.
+///
+/// Section keys mirror the server's response shape so the encode /
+/// decode round-trip is straightforward — the table stores
+/// (section, position, entityType, entityId), and toResult reassembles
+/// the parallel ID lists.
+final homeIndexProvider = StreamProvider<HomeIndex>((ref) {
+  final db = ref.watch(appDbProvider);
+  final query = (db.select(db.cachedHomeIndex)
+        ..orderBy([
+          (t) => drift.OrderingTerm.asc(t.section),
+          (t) => drift.OrderingTerm.asc(t.position),
+        ]))
+      .watch();
+  return cacheFirst<CachedHomeIndexData, HomeIndex>(
+    driftStream: query,
+    fetchAndPopulate: () async {
+      final api = await ref.read(libraryApiProvider.future);
+      final fresh = await api.getHomeIndex();
+      // Full replace in a transaction so the watch() sees exactly one
+      // post-fetch emission with the merged state. Section ordering is
+      // re-asserted on read (orderBy above) so the write order doesn't
+      // matter — letting us batch flat instead of section-by-section.
+      await db.transaction(() async {
+        await db.delete(db.cachedHomeIndex).go();
+        await db.batch((b) {
+          void rows(String section, String entityType, List<String> ids) {
+            for (var i = 0; i < ids.length; i++) {
+              b.insert(
+                db.cachedHomeIndex,
+                CachedHomeIndexCompanion.insert(
+                  section: section,
+                  position: i,
+                  entityType: entityType,
+                  entityId: ids[i],
+                ),
+              );
+            }
+          }
+
+          rows('recently_added_albums', 'album', fresh.recentlyAddedAlbums);
+          rows('rediscover_albums', 'album', fresh.rediscoverAlbums);
+          rows('rediscover_artists', 'artist', fresh.rediscoverArtists);
+          rows('most_played_tracks', 'track', fresh.mostPlayedTracks);
+          rows('last_played_artists', 'artist', fresh.lastPlayedArtists);
+        });
+      });
+    },
+    toResult: (rows) {
+      final recentlyAddedAlbums = <String>[];
+      final rediscoverAlbums = <String>[];
+      final rediscoverArtists = <String>[];
+      final mostPlayedTracks = <String>[];
+      final lastPlayedArtists = <String>[];
+      for (final r in rows) {
+        switch (r.section) {
+          case 'recently_added_albums':
+            recentlyAddedAlbums.add(r.entityId);
+          case 'rediscover_albums':
+            rediscoverAlbums.add(r.entityId);
+          case 'rediscover_artists':
+            rediscoverArtists.add(r.entityId);
+          case 'most_played_tracks':
+            mostPlayedTracks.add(r.entityId);
+          case 'last_played_artists':
+            lastPlayedArtists.add(r.entityId);
+        }
+      }
+      return HomeIndex(
+        recentlyAddedAlbums: recentlyAddedAlbums,
+        rediscoverAlbums: rediscoverAlbums,
+        rediscoverArtists: rediscoverArtists,
+        mostPlayedTracks: mostPlayedTracks,
+        lastPlayedArtists: lastPlayedArtists,
+      );
+    },
+    isOnline: () async => (await ref
+        .read(connectivityProvider.future)
+        .timeout(const Duration(seconds: 3), onTimeout: () => true)),
+    // SWR: cached layout shows instantly; fresh /api/home/index lands
+    // in the background and tiles re-resolve as the table mutates.
+    alwaysRefresh: true,
+    tag: 'homeIndex',
+  );
+});
+
 /// Encodes HomeData back to the wire-format JSON shape /api/home
 /// emits, so HomeData.fromJson can round-trip through the drift cache.
 String _encodeHomeData(HomeData h) => jsonEncode({