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/discover/discover_screen.dart

@@ -208,7 +208,7 @@ class _ResultTile extends StatelessWidget {
                 : CachedNetworkImage(
                     imageUrl: row.imageUrl,
                     fit: BoxFit.cover,
-                    fadeInDuration: Duration.zero,
+                    fadeInDuration: const Duration(milliseconds: 120),
                     fadeOutDuration: Duration.zero,
                     errorWidget: (_, __, ___) =>
                         Icon(Icons.album, color: fs.ash),