feat(flutter): replace Download with Regenerate on system playlist detail

Per operator decision: in the playlist detail header, system
playlists (For You / Discover) now show a Regenerate button where
user playlists keep Download. Offline-download is intentionally
dropped for system playlists — operator chose the literal swap.

- Regenerate calls PlaylistsApi.refreshSystem(variant), invalidates
  playlistsListProvider (home row tile rebinds to the rotated UUID),
  and pushReplacement's to /playlists/<newId> so the open detail
  screen rebinds instead of 404-ing on the stale id.
- Null id (empty library) and errors surface as snackbars.
- User playlists are unchanged (Download + Play).

The home-card kebab (#416, 7a04370) stays — web has refresh in both
the detail view and the home tile, so this matches web parity.

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

flutter_client/lib/playlists/playlist_detail_screen.dart

@@ -263,24 +263,32 @@ class _Header extends ConsumerWidget {
           ),
           const Spacer(),
           if (playable.isNotEmpty) ...[
-            OutlinedButton.icon(
-              key: const Key('download_playlist_button'),
-              onPressed: () {
-                final mgr = ref.read(audioCacheManagerProvider);
-                for (final t in playable) {
-                  final id = t.trackId ?? '';
-                  if (id.isEmpty) continue;
-                  // Fire-and-forget; downloads happen in background.
-                  // ignore: unawaited_futures
-                  mgr.pin(id, source: CacheSource.autoPlaylist);
-                }
-                ScaffoldMessenger.of(context).showSnackBar(
-                  SnackBar(content: Text('Downloading ${playable.length} tracks…')),
-                );
-              },
-              icon: const Icon(Icons.download, size: 16),
-              label: const Text('Download'),
-            ),
+            if (p.isSystem)
+              OutlinedButton.icon(
+                key: const Key('regenerate_playlist_button'),
+                onPressed: () => _regenerate(context, ref),
+                icon: const Icon(Icons.refresh, size: 16),
+                label: const Text('Regenerate'),
+              )
+            else
+              OutlinedButton.icon(
+                key: const Key('download_playlist_button'),
+                onPressed: () {
+                  final mgr = ref.read(audioCacheManagerProvider);
+                  for (final t in playable) {
+                    final id = t.trackId ?? '';
+                    if (id.isEmpty) continue;
+                    // Fire-and-forget; downloads happen in background.
+                    // ignore: unawaited_futures
+                    mgr.pin(id, source: CacheSource.autoPlaylist);
+                  }
+                  ScaffoldMessenger.of(context).showSnackBar(
+                    SnackBar(content: Text('Downloading ${playable.length} tracks…')),
+                  );
+                },
+                icon: const Icon(Icons.download, size: 16),
+                label: const Text('Download'),
+              ),
             const SizedBox(width: 8),
             FilledButton.icon(
               onPressed: () {
@@ -299,6 +307,37 @@ class _Header extends ConsumerWidget {
       ]),
     );
   }
+
+  /// Forces a server-side rebuild of this system playlist. The
+  /// rebuild rotates the playlist UUID, so the old detail route now
+  /// 404s — rebind by pushReplacement-ing to the new id. The
+  /// aggregate list is invalidated too so the home row's tile points
+  /// at the fresh UUID. ScaffoldMessenger + router captured before
+  /// the await so we don't touch a stale BuildContext after.
+  Future<void> _regenerate(BuildContext context, WidgetRef ref) async {
+    final messenger = ScaffoldMessenger.of(context);
+    final router = GoRouter.of(context);
+    final p = detail.playlist;
+    try {
+      final api = await ref.read(playlistsApiProvider.future);
+      final newId = await api.refreshSystem(p.systemVariant!);
+      ref.invalidate(playlistsListProvider);
+      if (newId == null) {
+        messenger.showSnackBar(const SnackBar(
+          content: Text('Nothing to build yet — library is empty.'),
+        ));
+        return;
+      }
+      messenger.showSnackBar(
+        SnackBar(content: Text('${p.name} regenerated')),
+      );
+      router.pushReplacement('/playlists/$newId');
+    } catch (e) {
+      messenger.showSnackBar(
+        SnackBar(content: Text("Couldn't regenerate: $e")),
+      );
+    }
+  }
 }
 
 class _PlaylistTrackRow extends ConsumerWidget {