feat(flutter/track-actions): AddToPlaylistSheet picker

Modal bottom sheet listing the caller's user-created playlists (system
playlists like For-You are filtered out — they're not user-mutable).
Tap a row to pop with the playlist id; consumer (TrackActionsSheet)
then calls PlaylistsApi.appendTracks.

Empty-state copy: "You haven't created any playlists yet."

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

flutter_client/lib/shared/widgets/track_actions/add_to_playlist_sheet.dart

@@ -0,0 +1,94 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../../playlists/playlists_provider.dart';
+import '../../../theme/theme_extension.dart';
+
+/// Modal bottom sheet listing the caller's user-created playlists.
+/// Returns the picked playlistId via Navigator.pop, or null on cancel.
+class AddToPlaylistSheet extends ConsumerWidget {
+  const AddToPlaylistSheet({super.key});
+
+  static Future<String?> show(BuildContext context) {
+    return showModalBottomSheet<String>(
+      context: context,
+      isScrollControlled: true,
+      builder: (_) => const AddToPlaylistSheet(),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final lists = ref.watch(playlistsListProvider('user'));
+    return SafeArea(
+      child: Container(
+        color: fs.iron,
+        padding: const EdgeInsets.symmetric(vertical: 8),
+        constraints: BoxConstraints(
+          maxHeight: MediaQuery.of(context).size.height * 0.6,
+        ),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            Padding(
+              padding: const EdgeInsets.fromLTRB(20, 8, 20, 8),
+              child: Text(
+                'Add to playlist',
+                style: TextStyle(
+                  color: fs.parchment,
+                  fontFamily: 'Fraunces',
+                  fontSize: 20,
+                ),
+              ),
+            ),
+            Flexible(
+              child: lists.when(
+                loading: () => const Center(child: CircularProgressIndicator()),
+                error: (e, _) => Padding(
+                  padding: const EdgeInsets.all(16),
+                  child: Text('$e', style: TextStyle(color: fs.error)),
+                ),
+                data: (data) {
+                  final owned = data.owned
+                      .where((p) => p.systemVariant == null)
+                      .toList();
+                  if (owned.isEmpty) {
+                    return Padding(
+                      padding: const EdgeInsets.all(20),
+                      child: Text(
+                        "You haven't created any playlists yet.",
+                        style: TextStyle(color: fs.ash),
+                      ),
+                    );
+                  }
+                  return ListView.builder(
+                    shrinkWrap: true,
+                    itemCount: owned.length,
+                    itemBuilder: (_, i) {
+                      final p = owned[i];
+                      return ListTile(
+                        key: Key('add_to_playlist_${p.id}'),
+                        leading: Icon(Icons.queue_music, color: fs.parchment),
+                        title: Text(
+                          p.name,
+                          style: TextStyle(color: fs.parchment),
+                        ),
+                        subtitle: Text(
+                          '${p.trackCount} ${p.trackCount == 1 ? "track" : "tracks"}',
+                          style: TextStyle(color: fs.ash, fontSize: 12),
+                        ),
+                        onTap: () => Navigator.pop(context, p.id),
+                      );
+                    },
+                  );
+                },
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}

flutter_client/test/shared/widgets/track_actions/add_to_playlist_sheet_test.dart

@@ -0,0 +1,114 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:minstrel/api/endpoints/playlists.dart';
+import 'package:minstrel/models/playlist.dart';
+import 'package:minstrel/playlists/playlists_provider.dart';
+import 'package:minstrel/shared/widgets/track_actions/add_to_playlist_sheet.dart';
+import 'package:minstrel/theme/theme_data.dart';
+
+const _userPlaylist = Playlist(
+  id: 'p1',
+  userId: 'u1',
+  name: 'Road trip',
+  description: '',
+  isPublic: false,
+  systemVariant: null,
+  trackCount: 12,
+  coverUrl: '',
+  ownerUsername: 'alice',
+  createdAt: '2026-05-01T00:00:00Z',
+  updatedAt: '2026-05-01T00:00:00Z',
+);
+
+const _systemPlaylist = Playlist(
+  id: 'p2',
+  userId: 'u1',
+  name: 'For You',
+  description: '',
+  isPublic: false,
+  systemVariant: 'for_you',
+  trackCount: 75,
+  coverUrl: '',
+  ownerUsername: 'alice',
+  createdAt: '2026-05-01T00:00:00Z',
+  updatedAt: '2026-05-01T00:00:00Z',
+);
+
+Widget _harness(PlaylistsList lists) {
+  return ProviderScope(
+    overrides: [
+      playlistsListProvider('user').overrideWith((ref) async => lists),
+    ],
+    child: MaterialApp(
+      theme: buildThemeData(),
+      home: Builder(builder: (ctx) {
+        return Scaffold(
+          body: Center(
+            child: ElevatedButton(
+              onPressed: () => AddToPlaylistSheet.show(ctx),
+              child: const Text('open'),
+            ),
+          ),
+        );
+      }),
+    ),
+  );
+}
+
+void main() {
+  testWidgets('renders user playlists, hides system ones', (t) async {
+    await t.pumpWidget(_harness(
+      const PlaylistsList(owned: [_userPlaylist, _systemPlaylist], public: []),
+    ));
+    await t.tap(find.text('open'));
+    await t.pumpAndSettle();
+    expect(find.text('Road trip'), findsOneWidget);
+    expect(find.text('For You'), findsNothing);
+  });
+
+  testWidgets('empty state when no user playlists', (t) async {
+    await t.pumpWidget(_harness(
+      const PlaylistsList(owned: [_systemPlaylist], public: []),
+    ));
+    await t.tap(find.text('open'));
+    await t.pumpAndSettle();
+    expect(
+      find.text("You haven't created any playlists yet."),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets('tapping a row pops with playlist id', (t) async {
+    String? picked;
+    await t.pumpWidget(ProviderScope(
+      overrides: [
+        playlistsListProvider('user').overrideWith(
+          (ref) async =>
+              const PlaylistsList(owned: [_userPlaylist], public: []),
+        ),
+      ],
+      child: MaterialApp(
+        theme: buildThemeData(),
+        home: Builder(builder: (ctx) {
+          return Scaffold(
+            body: Center(
+              child: ElevatedButton(
+                onPressed: () async {
+                  picked = await AddToPlaylistSheet.show(ctx);
+                },
+                child: const Text('open'),
+              ),
+            ),
+          );
+        }),
+      ),
+    ));
+    await t.tap(find.text('open'));
+    await t.pumpAndSettle();
+    await t.tap(find.byKey(const Key('add_to_playlist_p1')));
+    await t.pumpAndSettle();
+    expect(picked, 'p1');
+  });
+}