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/cache/tile_providers.dart

@@ -87,11 +87,8 @@ final artistTileProvider =
   }
 });
 
-/// Watches the cached_tracks row for [id]. No hydration today — see
-/// hydration_queue.dart's `case 'track':` note. The tile renders a
-/// skeleton while drift is empty; once sync populates the row (or a
-/// future GET /api/tracks/:id endpoint enables real hydration), the
-/// stream emits the populated TrackRef.
+/// Watches the cached_tracks row for [id]. Triggers background
+/// hydration on miss; yields the row once it lands.
 final trackTileProvider =
     StreamProvider.family<TrackRef?, String>((ref, id) async* {
   if (id.isEmpty) {
@@ -106,8 +103,15 @@ final trackTileProvider =
     leftOuterJoin(db.cachedAlbums,
         db.cachedAlbums.id.equalsExp(db.cachedTracks.albumId)),
   ]);
+  var enqueued = false;
   await for (final rows in query.watch()) {
     if (rows.isEmpty) {
+      if (!enqueued) {
+        enqueued = true;
+        ref
+            .read(hydrationQueueProvider)
+            .enqueue(entityType: 'track', entityId: id);
+      }
       yield null;
       continue;
     }