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>

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