feat(flutter): playlist cover art via deterministic /api/playlists/{id}/cover

Same fix shape as albums: drift's CachedPlaylistAdapter.toRef was
returning coverUrl: '' because the cache table doesn't persist the
server-derived URL. Set it to /api/playlists/<id>/cover in the
adapter — handleGetPlaylistCover serves the cached collage from
disk, so the URL is deterministic and the round-trip through drift
no longer drops it.

PlaylistCard + PlaylistsListScreen pass the existing queue_music
icon as ServerImage's fallback, so when the server hasn't built a
collage yet (system playlists with no tracks at build time), the
endpoint 404s and the icon shows over the slate background instead
of an empty box.

flutter_client/lib/cache/adapters.dart

@@ -88,6 +88,12 @@ extension TrackRefDriftWrite on TrackRef {
 extension CachedPlaylistAdapter on CachedPlaylist {
   /// `ownerUsername` is server-derived; cache stores the userId only.
   /// Pass empty unless a join supplies it.
+  /// `coverUrl` is reconstructed deterministically from the playlist
+  /// id — the server serves the cached collage at this path
+  /// (handleGetPlaylistCover). Mirrors the album-cover trick. When
+  /// the server hasn't built a collage yet (system playlists with no
+  /// tracks at build time), the endpoint 404s and PlaylistCard's
+  /// ServerImage falls back to its slate placeholder.
   Playlist toRef({String ownerUsername = ''}) => Playlist(
         id: id,
         userId: userId,
@@ -96,7 +102,7 @@ extension CachedPlaylistAdapter on CachedPlaylist {
         isPublic: isPublic,
         systemVariant: systemVariant,
         trackCount: trackCount,
-        coverUrl: '', // server-derived; cache doesn't persist
+        coverUrl: '/api/playlists/$id/cover',
         ownerUsername: ownerUsername,
         createdAt: '',
         updatedAt: '',

flutter_client/lib/playlists/playlists_list_screen.dart

@@ -83,7 +83,11 @@ class _PlaylistTile extends StatelessWidget {
               color: fs.slate,
               child: playlist.coverUrl.isEmpty
                   ? Icon(Icons.queue_music, color: fs.ash)
-                  : ServerImage(url: playlist.coverUrl, fit: BoxFit.cover),
+                  : ServerImage(
+                      url: playlist.coverUrl,
+                      fit: BoxFit.cover,
+                      fallback: Icon(Icons.queue_music, color: fs.ash),
+                    ),
             ),
           ),
           const SizedBox(width: 12),

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -31,9 +31,19 @@ class PlaylistCard extends StatelessWidget {
                   width: 144,
                   height: 144,
                   color: fs.slate,
+                  // coverUrl is deterministic per playlist (see
+                  // CachedPlaylistAdapter.toRef). When the server's
+                  // collage isn't built yet, ServerImage's
+                  // errorBuilder shows the queue_music icon over the
+                  // slate background.
                   child: playlist.coverUrl.isEmpty
                       ? Icon(Icons.queue_music, color: fs.ash, size: 56)
-                      : ServerImage(url: playlist.coverUrl, fit: BoxFit.cover),
+                      : ServerImage(
+                          url: playlist.coverUrl,
+                          fit: BoxFit.cover,
+                          fallback:
+                              Icon(Icons.queue_music, color: fs.ash, size: 56),
+                        ),
                 ),
               ),
               const SizedBox(height: 8),