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, 14 insertions, 0 deletions

diff --git a/player-android/lib/utils/error_mappers.dart b/player-android/lib/utils/error_mappers.dart
index 5a9996a..7353928 100644
--- a/player-android/lib/utils/error_mappers.dart
+++ b/player-android/lib/utils/error_mappers.dart
@@ -77,3 +77,17 @@ String setsErrorMessage(Object error) {
   }
   return 'Unexpected error. Please try again.';
 }
+
+/// Maps any thrown object from [PlayerApiClient.listMedia] to a UI string.
+///
+/// Identical delegation strategy to [setsErrorMessage]: DioExceptions are
+/// mapped by [dioConnectionErrorMessage]; all other exceptions fall back to a
+/// generic message.  Having a separate function preserves the option to add
+/// media-specific status-code overrides (e.g. 403 permission errors) later
+/// without altering the sets helper (Open-Closed Principle).
+String mediaErrorMessage(Object error) {
+  if (error is DioException) {
+    return dioConnectionErrorMessage(error);
+  }
+  return 'Unexpected error. Please try again.';
+}