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/artist_card.dart

@@ -1,17 +1,23 @@
+import 'dart:math';
+
 import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 
 import '../../models/artist.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 ArtistCard extends StatelessWidget {
+class ArtistCard extends ConsumerWidget {
   const ArtistCard({required this.artist, required this.onTap, super.key});
   final ArtistRef artist;
   final VoidCallback onTap;
 
   @override
-  Widget build(BuildContext context) {
+  Widget build(BuildContext context, WidgetRef ref) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
     return SizedBox(
       width: 140,
@@ -22,16 +28,31 @@ class ArtistCard extends StatelessWidget {
           child: Padding(
             padding: const EdgeInsets.symmetric(horizontal: 8),
             child: Column(crossAxisAlignment: CrossAxisAlignment.start, children: [
-              ClipOval(
-                child: Container(
-                  width: 124,
-                  height: 124,
-                  color: fs.slate,
-                  child: artist.coverUrl.isEmpty
-                      ? SvgPicture.asset('assets/svg/album-fallback.svg',
-                          fit: BoxFit.cover)
-                      : ServerImage(url: artist.coverUrl, fit: BoxFit.cover),
-                ),
+              // Stack: circular avatar + overlaid play button at bottom-right.
+              // The avatar is a circle (ClipOval) — bottom-right of its
+              // bounding box still places the button inside the visible area
+              // because the button is small relative to the radius.
+              Stack(
+                children: [
+                  ClipOval(
+                    child: Container(
+                      width: 124,
+                      height: 124,
+                      color: fs.slate,
+                      child: artist.coverUrl.isEmpty
+                          ? SvgPicture.asset('assets/svg/album-fallback.svg',
+                              fit: BoxFit.cover)
+                          : ServerImage(url: artist.coverUrl, fit: BoxFit.cover),
+                    ),
+                  ),
+                  Positioned(
+                    bottom: 4,
+                    right: 4,
+                    child: PlayCircleButton(
+                      onPressed: () => _playArtistShuffle(ref),
+                    ),
+                  ),
+                ],
               ),
               const SizedBox(height: 8),
               Text(
@@ -46,4 +67,18 @@ class ArtistCard extends StatelessWidget {
       ),
     );
   }
+
+  /// Fetches the artist's tracks via /api/artists/{id}/tracks, shuffles
+  /// them (Fisher-Yates, default Random), and plays from index 0.
+  /// Matches the web ArtistCard's `playQueue(shuffle(tracks), 0)`.
+  Future<void> _playArtistShuffle(WidgetRef ref) async {
+    final api = await ref.read(libraryApiProvider.future);
+    final tracks = await api.getArtistTracks(artist.id);
+    if (tracks.isEmpty) return;
+    final shuffled = List.of(tracks);
+    shuffled.shuffle(Random());
+    await ref
+        .read(playerActionsProvider)
+        .playTracks(shuffled, initialIndex: 0);
+  }
 }