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/models/home_index.dart

@@ -0,0 +1,41 @@
+/// Mirrors internal/api/types.go HomeIndexPayload. Five flat slices of
+/// entity ID strings — the per-item rendering variant of HomeData.
+/// Section name implies entity type; no per-entry type tag is needed.
+///
+/// Slices are non-null after fromJson so callers can branch on `length`
+/// instead of dealing with null sections.
+class HomeIndex {
+  const HomeIndex({
+    required this.recentlyAddedAlbums,
+    required this.rediscoverAlbums,
+    required this.rediscoverArtists,
+    required this.mostPlayedTracks,
+    required this.lastPlayedArtists,
+  });
+
+  final List<String> recentlyAddedAlbums;
+  final List<String> rediscoverAlbums;
+  final List<String> rediscoverArtists;
+  final List<String> mostPlayedTracks;
+  final List<String> lastPlayedArtists;
+
+  static const empty = HomeIndex(
+    recentlyAddedAlbums: [],
+    rediscoverAlbums: [],
+    rediscoverArtists: [],
+    mostPlayedTracks: [],
+    lastPlayedArtists: [],
+  );
+
+  factory HomeIndex.fromJson(Map<String, dynamic> j) {
+    List<String> ids(String key) =>
+        ((j[key] as List?) ?? const []).map((e) => e.toString()).toList();
+    return HomeIndex(
+      recentlyAddedAlbums: ids('recently_added_albums'),
+      rediscoverAlbums: ids('rediscover_albums'),
+      rediscoverArtists: ids('rediscover_artists'),
+      mostPlayedTracks: ids('most_played_tracks'),
+      lastPlayedArtists: ids('last_played_artists'),
+    );
+  }
+}