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, 11 insertions, 0 deletions

diff --git a/player-android/lib/api/player_api_client.dart b/player-android/lib/api/player_api_client.dart
index 13b5ead..9e67d9e 100644
--- a/player-android/lib/api/player_api_client.dart
+++ b/player-android/lib/api/player_api_client.dart
@@ -133,6 +133,17 @@ class PlayerApiClient {
 
   Future<Uint8List> getThumbnail(int mediaId) => throw UnimplementedError();
 
+  /// Returns the URL for a media item's thumbnail image.
+  ///
+  /// Constructing the URL here (rather than in the screen layer) keeps the API
+  /// path constant `/api/v1/media/{id}/thumbnail` in one place and avoids
+  /// exposing [Dio] or its [BaseOptions] to the UI layer (Dependency Inversion).
+  ///
+  /// The base URL is derived from the underlying Dio instance so it is always
+  /// consistent with the rest of the API calls.
+  String thumbnailUrl(int mediaId) =>
+      '${rawDio.options.baseUrl}/api/v1/media/$mediaId/thumbnail';
+
   Future<void> regenerateThumbnail(int mediaId) => throw UnimplementedError();
 
   Future<bool> toggleFavorite(int mediaId) => throw UnimplementedError();