feat(flutter): cosmetic reveal animations (Slice F)

Final slice of the per-item rendering pass. Wraps every tile widget
in an AnimatedSwitcher between the skeleton placeholder and the real
card. 220ms cross-fade with easeOut: tiles "settle into place"
rather than hard-cutting from shimmer to content. Since each tile
fades independently as its data lands — and the HydrationQueue's
concurrency cap drains in a natural cascade — the overall feel is
the staged "page builds piece by piece" effect we wanted, with no
per-tile position math required.

Bumps CachedNetworkImage fadeInDuration from zero to 120ms (server_
image.dart, discover_screen.dart). Imperceptible on cache hits since
the image decodes synchronously; on cache misses the bytes fade in
smoothly instead of popping. Slice 1's "zero fade" call was right
about the 500ms default being a regression, but 120ms threads the
needle.

Playlist detail wraps its body in the same AnimatedSwitcher so the
cold-load skeleton page cross-fades into the real track list.

Tiles affected: home _AlbumTile / _ArtistTile / _TrackTile + liked
_LikedAlbumTile / _LikedArtistTile / _LikedTrackRow + playlist
_SkeletonBody / _Body. All keyed via ValueKey so AnimatedSwitcher
detects the transition.

End of the per-item pass. Net behavior: cold visits paint shaped
pages instantly with skeletons, content cascades in as hydration
lands; warm visits paint fully from drift in the first frame.

flutter_client/lib/playlists/playlist_detail_screen.dart

@@ -67,16 +67,23 @@ class PlaylistDetailScreen extends ConsumerWidget {
                 overflow: TextOverflow.ellipsis,
               ),
       ),
-      body: detail.when(
-        // Loading from a seed: render the header immediately + the
-        // exact track-count of skeleton rows, instead of an opaque
-        // full-screen spinner. Cold-visit feel: shaped page with
-        // shimmering rows that swap in for real ones as the bulk
-        // detail fetch lands.
-        loading: () => _SkeletonBody(seed: seed),
-        error: (e, _) =>
-            Center(child: Text('$e', style: TextStyle(color: fs.error))),
-        data: (d) => _Body(detail: d),
+      // AnimatedSwitcher between the skeleton body and the real body
+      // smooths the cold-visit moment when bulk detail lands. 220ms
+      // matches the per-tile reveal feel used on home / liked tabs.
+      body: AnimatedSwitcher(
+        duration: const Duration(milliseconds: 220),
+        switchInCurve: Curves.easeOut,
+        child: detail.when(
+          loading: () => _SkeletonBody(
+            key: const ValueKey('skeleton'),
+            seed: seed,
+          ),
+          error: (e, _) => Center(
+            key: const ValueKey('error'),
+            child: Text('$e', style: TextStyle(color: fs.error)),
+          ),
+          data: (d) => _Body(key: const ValueKey('body'), detail: d),
+        ),
       ),
     );
   }
@@ -88,7 +95,7 @@ class PlaylistDetailScreen extends ConsumerWidget {
 /// (deep link straight to a playlist with no prior cache) the
 /// skeleton renders a small default and grows when real data arrives.
 class _SkeletonBody extends StatelessWidget {
-  const _SkeletonBody({this.seed});
+  const _SkeletonBody({super.key, this.seed});
   final Playlist? seed;
 
   static const _defaultSkeletonCount = 8;
@@ -163,7 +170,7 @@ class _PlaylistTrackSkeleton extends StatelessWidget {
 }
 
 class _Body extends ConsumerWidget {
-  const _Body({required this.detail});
+  const _Body({super.key, required this.detail});
   final PlaylistDetail detail;
 
   @override