feat(flutter): #393 — always-visible play button on home cards

AlbumCard / ArtistCard / PlaylistCard gain a 44dp circular play
button overlaid bottom-right of the cover art. Mirrors the
hover-revealed .play-overlay on the web cards; always visible
because hover is not a real interaction on touch.

Per-card semantics match the web:

- AlbumCard: fetches /api/albums/{id}, starts playback from track 0.
- ArtistCard: fetches /api/artists/{id}/tracks, Fisher-Yates shuffles,
  plays from index 0 (matches web's playQueue(shuffle(tracks), 0)).
- PlaylistCard: fetches /api/playlists/{id}, materializes available
  rows into TrackRef, plays from index 0. Disabled state when
  trackCount == 0 — semi-transparent button, taps ignored.

Shared PlayCircleButton widget manages loading state (spinner during
fetch) so each card's onPressed can stay async without re-entrancy
guards. Cards become ConsumerWidget so they can reach the
playerActionsProvider + relevant API providers; constructor surface
unchanged so existing call sites (artist_detail, library_screen,
home_screen) keep working.

CompactTrackCard unchanged — its tap already plays-from-here.

For #393 / #356 umbrella.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/library/widgets/album_card.dart

@@ -1,11 +1,15 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 
 import '../../models/album.dart';
+import '../../player/player_provider.dart';
 import '../../shared/widgets/server_image.dart';
 import '../../theme/theme_extension.dart';
+import '../library_providers.dart';
+import 'play_circle_button.dart';
 
-class AlbumCard extends StatelessWidget {
+class AlbumCard extends ConsumerWidget {
   const AlbumCard({
     required this.album,
     required this.onTap,
@@ -32,7 +36,7 @@ class AlbumCard extends StatelessWidget {
   final bool showArtist;
 
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context, WidgetRef ref) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
     final coverSize = width - 16;
     return SizedBox(
@@ -47,17 +51,29 @@ class AlbumCard extends StatelessWidget {
               crossAxisAlignment: CrossAxisAlignment.start,
               mainAxisSize: MainAxisSize.min,
               children: [
-                ClipRRect(
-                  borderRadius: BorderRadius.circular(6),
-                  child: Container(
-                    width: coverSize,
-                    height: coverSize,
-                    color: fs.slate,
-                    child: album.coverUrl.isEmpty
-                        ? SvgPicture.asset('assets/svg/album-fallback.svg',
-                            fit: BoxFit.cover)
-                        : ServerImage(url: album.coverUrl, fit: BoxFit.cover),
-                  ),
+                // Stack: cover image + overlaid play button at bottom-right.
+                Stack(
+                  children: [
+                    ClipRRect(
+                      borderRadius: BorderRadius.circular(6),
+                      child: Container(
+                        width: coverSize,
+                        height: coverSize,
+                        color: fs.slate,
+                        child: album.coverUrl.isEmpty
+                            ? SvgPicture.asset('assets/svg/album-fallback.svg',
+                                fit: BoxFit.cover)
+                            : ServerImage(url: album.coverUrl, fit: BoxFit.cover),
+                      ),
+                    ),
+                    Positioned(
+                      bottom: 6,
+                      right: 6,
+                      child: PlayCircleButton(
+                        onPressed: () => _playAlbum(ref),
+                      ),
+                    ),
+                  ],
                 ),
                 const SizedBox(height: 8),
                 Text(
@@ -80,4 +96,17 @@ class AlbumCard extends StatelessWidget {
       ),
     );
   }
+
+  /// Fetches the album's tracks via /api/albums/{id} and starts playback
+  /// from the first track. Errors are swallowed by the button's outer
+  /// try/finally; callers don't surface them — failed fetches just keep
+  /// the spinner visible until the button is retapped.
+  Future<void> _playAlbum(WidgetRef ref) async {
+    final api = await ref.read(libraryApiProvider.future);
+    final result = await api.getAlbum(album.id);
+    if (result.tracks.isEmpty) return;
+    await ref
+        .read(playerActionsProvider)
+        .playTracks(result.tracks, initialIndex: 0);
+  }
 }