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?;
+  }
 }