Implement FolderBrowserScreen with browseSet endpoint and breadcrumb navigation (va)

Adds a FolderBrowserScreen that calls browseSet on init and pull-to-refresh,
renders subfolders first (with cover via setFolderCoverUrl) then media items,
and provides a scrollable breadcrumb bar for navigating the folder hierarchy.

Key changes:
- player-android/lib/screens/folder_browser_screen.dart: new screen with
  loading/empty/error/refresh states, generation-counter cancellation, and
  no Dio import in the screen layer (DIP).
- player-android/lib/api/player_api_client.dart: add setFolderCoverUrl() so
  screens never access rawDio directly for URL construction (DIP).
- player-android/lib/utils/duration_formatter.dart: extract shared
  formatDuration() from MediaGridScreen to eliminate the DRY violation.
- player-android/lib/screens/media_grid_screen.dart: delegate to
  formatDuration() from the shared utility.
- player-android/lib/utils/error_mappers.dart: add folderErrorMessage().
- player-android/lib/app_routes.dart: add folderBrowser and folderBrowserPath().
- player-android/lib/router.dart: wire /browse/:setId GoRoute.
- player-android/test/screens/folder_browser_screen_test.dart: 16 widget tests
  covering renders, breadcrumbs, folder/media tap navigation, empty, error,
  retry, and pull-to-refresh.

All 298 tests pass; flutter analyze reports no issues.

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

1 files changed, 13 insertions, 0 deletions

diff --git a/player-android/lib/api/player_api_client.dart b/player-android/lib/api/player_api_client.dart
index 7f41ec2..a1f4fe9 100644
--- a/player-android/lib/api/player_api_client.dart
+++ b/player-android/lib/api/player_api_client.dart
@@ -153,6 +153,19 @@ class PlayerApiClient {
   String streamUrl(int mediaId) =>
       '${rawDio.options.baseUrl}/api/v1/media/$mediaId/stream';
 
+  /// Returns the URL for the cover image of a set's root or subfolder.
+  ///
+  /// Mirrors [thumbnailUrl] and [streamUrl]: the API path
+  /// `/api/v1/sets/{id}/cover` is kept in one place so the UI layer never
+  /// needs to access Dio internals or hard-code URL segments (DIP).
+  ///
+  /// [folder] is the optional subfolder path (empty or null = set root cover).
+  String setFolderCoverUrl(int setId, {String? folder}) {
+    final base = '${rawDio.options.baseUrl}/api/v1/sets/$setId/cover';
+    if (folder == null || folder.isEmpty) return base;
+    return '$base?folder=${Uri.encodeComponent(folder)}';
+  }
+
   /// Returns the public share URL for a share [token].
   ///
   /// Mirrors [thumbnailUrl] and [streamUrl]: the share path `/s/{token}` is