feat(flutter): refresh kebab on system playlist cards (#416 parity)

Closes the Flutter half of Fable #416. Web got a generalized
system-playlist refresh kebab in d12afda; this brings Flutter to
parity instead of leaving the affordance web-only.

- PlaylistsApi.refreshSystem(variant): POST
  /api/playlists/system/{for-you|discover}/refresh, maps the
  underscore model variant to the hyphenated route segment,
  returns the rotated playlist id.
- PlaylistCard: top-right PopupMenuButton on system playlists
  with a context-labelled "Refresh For You" / "Refresh Discover"
  item. Calls refreshSystem, invalidates playlistsListProvider
  (which reconciles the rotated UUID + new tracks), snackbars
  the result. ScaffoldMessenger captured pre-await.
- Tests: kebab present for system, absent for user playlists.

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

flutter_client/lib/api/endpoints/playlists.dart

@@ -53,4 +53,20 @@ class PlaylistsApi {
       data: {'track_ids': trackIds},
     );
   }
+
+  /// POST /api/playlists/system/{variant}/refresh. Synchronously
+  /// rebuilds the caller's system playlist for the given variant
+  /// ("for_you" | "discover") and returns the new playlist id, or
+  /// null when the library is empty so there's nothing to build.
+  ///
+  /// The variant uses underscores in the model (system_variant) but
+  /// the route segment is hyphenated ("for-you"), so map here.
+  Future<String?> refreshSystem(String variant) async {
+    final segment = variant == 'for_you' ? 'for-you' : variant;
+    final r = await _dio.post<Map<String, dynamic>>(
+      '/api/playlists/system/$segment/refresh',
+      data: const <String, dynamic>{},
+    );
+    return (r.data ?? const {})['playlist_id'] as String?;
+  }
 }

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -66,6 +66,31 @@ class PlaylistCard extends ConsumerWidget {
                       onPressed: () => _playPlaylist(ref),
                     ),
                   ),
+                  // System playlists get a refresh affordance so the
+                  // user can force a fresh mix instead of waiting for
+                  // the daily 03:00 rebuild. Mirrors the web kebab.
+                  if (playlist.isSystem)
+                    Positioned(
+                      top: 2,
+                      right: 2,
+                      child: PopupMenuButton<String>(
+                        icon: Icon(Icons.more_vert, color: fs.parchment, size: 18),
+                        tooltip: 'Playlist actions',
+                        color: fs.iron,
+                        onSelected: (_) => _refresh(context, ref),
+                        itemBuilder: (_) => [
+                          PopupMenuItem<String>(
+                            value: 'refresh',
+                            child: Row(children: [
+                              Icon(Icons.refresh, color: fs.parchment, size: 18),
+                              const SizedBox(width: 8),
+                              Text(_refreshLabel,
+                                  style: TextStyle(color: fs.parchment)),
+                            ]),
+                          ),
+                        ],
+                      ),
+                    ),
                 ],
               ),
               const SizedBox(height: 8),
@@ -98,6 +123,32 @@ class PlaylistCard extends ConsumerWidget {
     );
   }
 
+  String get _refreshLabel => switch (playlist.systemVariant) {
+        'for_you' => 'Refresh For You',
+        'discover' => 'Refresh Discover',
+        _ => 'Refresh',
+      };
+
+  /// Forces a server-side rebuild of this system playlist, then
+  /// invalidates the aggregate list so the rotated UUID + new tracks
+  /// land on the next read. ScaffoldMessenger is captured before the
+  /// await so we don't touch a stale BuildContext afterward.
+  Future<void> _refresh(BuildContext context, WidgetRef ref) async {
+    final messenger = ScaffoldMessenger.of(context);
+    try {
+      final api = await ref.read(playlistsApiProvider.future);
+      await api.refreshSystem(playlist.systemVariant!);
+      ref.invalidate(playlistsListProvider);
+      messenger.showSnackBar(
+        SnackBar(content: Text('${playlist.name} refreshed')),
+      );
+    } catch (e) {
+      messenger.showSnackBar(
+        SnackBar(content: Text("Couldn't refresh: $e")),
+      );
+    }
+  }
+
   /// Fetches the playlist via /api/playlists/{id}, materializes each
   /// PlaylistTrack into a TrackRef (filtering out unavailable rows
   /// whose `trackId` is null after a track-delete), and plays from

flutter_client/test/playlists/widgets/playlist_card_test.dart

@@ -55,4 +55,24 @@ void main() {
     expect(find.text('For You'), findsOneWidget);
     expect(find.text('for you'), findsOneWidget);
   });
+
+  testWidgets('shows refresh kebab on system playlists', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistCard(playlist: _forYou),
+      ),
+    ));
+    expect(find.byIcon(Icons.more_vert), findsOneWidget);
+  });
+
+  testWidgets('no refresh kebab on user playlists', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistCard(playlist: _userPlaylist),
+      ),
+    ));
+    expect(find.byIcon(Icons.more_vert), findsNothing);
+  });
 }