feat: add animation utilities and fix regressions in UI refactor

- Add animation_utils.dart with skeleton loaders, staggered list animations,
  animated checkboxes, badge bump, download success overlay, and shared
  page route helper
- Replace CircularProgressIndicator with shimmer skeleton loaders across
  album, artist, playlist, search, store, and extension screens
- Unify page transitions via slidePageRoute (MaterialPageRoute) for
  Android predictive back gesture support
- Extract AnimatedSelectionCheckbox with configurable unselectedColor
  to preserve original transparent/opaque backgrounds per context
- Add swipe-to-dismiss on download queue items with confirmDismiss
  dialog for active downloads to prevent accidental cancellation
- Add Hero animations for cover art transitions between list and detail
- Add AnimatedBadge bump on navigation bar badge count changes
- Add DownloadSuccessOverlay green flash on download completion
- Restore fine-grained ref.watch(.select()) in _CollectionTrackTile
  to avoid full list rebuilds on download history changes
- Fix DownloadSuccessOverlay re-flashing on widget recreation by
  initialising _wasSuccess from initial widget state
- Remove orphan Hero tag in search_screen that had no matching pair
- Chip borderRadius updated from 8 to 20 for consistency

lib/theme/app_theme.dart

@@ -89,9 +89,7 @@ class AppTheme {
 
   static CardThemeData _cardTheme(ColorScheme scheme) => CardThemeData(
     elevation: 0,
-    shape: RoundedRectangleBorder(
-      borderRadius: BorderRadius.circular(16),
-    ),
+    shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(16)),
     color: scheme.surfaceContainerLow,
     surfaceTintColor: scheme.surfaceTint,
   );
@@ -148,9 +146,7 @@ class AppTheme {
   static InputDecorationTheme _inputDecorationTheme(ColorScheme scheme) =>
       InputDecorationTheme(
         filled: true,
-        fillColor: scheme.surfaceContainerHighest.withValues(
-          alpha: 0.3,
-        ),
+        fillColor: scheme.surfaceContainerHighest.withValues(alpha: 0.3),
         border: OutlineInputBorder(
           borderRadius: BorderRadius.circular(16),
           borderSide: BorderSide.none,
@@ -175,9 +171,7 @@ class AppTheme {
 
   static ListTileThemeData _listTileTheme(ColorScheme scheme) =>
       ListTileThemeData(
-        shape: RoundedRectangleBorder(
-          borderRadius: BorderRadius.circular(16),
-        ),
+        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(16)),
         contentPadding: const EdgeInsets.symmetric(horizontal: 16, vertical: 4),
       );
 
@@ -237,7 +231,7 @@ class AppTheme {
   );
 
   static ChipThemeData _chipTheme(ColorScheme scheme) => ChipThemeData(
-    shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(8)),
+    shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(20)),
     backgroundColor: scheme.surfaceContainerLow,
     selectedColor: scheme.secondaryContainer,
   );