Implement MediaGridScreen with grid, loading/empty/error states, and widget tests (ua)

- Replace placeholder MediaGridScreen with a full implementation: loads
  media via listMedia(setId:), renders a Material 3 2-column grid of
  thumbnail cards with filename, type icon, and duration overlay.
- Add thumbnailUrl(int mediaId) to PlayerApiClient so screen files
  construct thumbnail URLs without importing Dio (DIP).
- Add mediaErrorMessage() top-level helper to error_mappers.dart
  (Open-Closed Principle; matches setsErrorMessage pattern).
- Update router.dart to forward the set name as a route extra so
  MediaGridScreen shows the name in the app bar without extra API calls.
- Update home_screen.dart _SetCard tap to pass set name as route extra.
- Add 10 widget tests covering: loading, grid render, tap navigation
  to /media/:id, empty state, error state + retry, and pull-to-refresh.
- All 109 tests pass; flutter analyze reports no issues.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/player-android/lib/router.dart b/player-android/lib/router.dart
index 4f1934c..ee101a5 100644
--- a/player-android/lib/router.dart
+++ b/player-android/lib/router.dart
@@ -106,7 +106,11 @@ final routerProvider = Provider<GoRouter>((ref) {
           // The ':setId' path parameter is guaranteed by the route pattern.
           final raw = state.pathParameters['setId']!;
           final setId = int.tryParse(raw) ?? 0;
-          return MediaGridScreen(setId: setId);
+          // The set name is optionally passed as a route extra (String) by the
+          // calling screen (e.g. SetsListScreen) so the app bar can show it
+          // immediately without an extra API call.
+          final setName = state.extra is String ? state.extra as String : null;
+          return MediaGridScreen(setId: setId, setName: setName);
         },
       ),
       GoRoute(