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, 29 insertions, 0 deletions

diff --git a/player-android/lib/utils/duration_formatter.dart b/player-android/lib/utils/duration_formatter.dart
new file mode 100644
index 0000000..72e810b
--- /dev/null
+++ b/player-android/lib/utils/duration_formatter.dart
@@ -0,0 +1,29 @@
+// Duration formatting utilities shared across screen widgets.
+//
+// Centralises the conversion of fractional seconds to a human-readable string
+// so that [MediaGridScreen] and [FolderBrowserScreen] do not each carry their
+// own copy of the same logic (DRY / Single Responsibility).
+//
+// All functions are pure data-transformations: no widget state, no Riverpod
+// reads, no BuildContext — making them easy to unit-test in isolation.
+
+/// Formats [seconds] as `h:mm:ss` or `m:ss`, omitting leading zeros in the
+/// hours and minutes positions.
+///
+/// Examples:
+///   - 7320.0 → "2:02:00"
+///   - 210.5  → "3:30"
+///   - 30.0   → "0:30"
+///
+/// Uses integer arithmetic only — no [Duration] dependency — so the helper
+/// is lightweight and independently testable.
+String formatDuration(double seconds) {
+  final total = seconds.truncate();
+  final h = total ~/ 3600;
+  final m = (total % 3600) ~/ 60;
+  final s = total % 60;
+  if (h > 0) {
+    return '$h:${m.toString().padLeft(2, '0')}:${s.toString().padLeft(2, '0')}';
+  }
+  return '$m:${s.toString().padLeft(2, '0')}';
+}