feat: add multi-select share and batch convert in downloaded/local album screens

- Add shareMultipleContentUris native handler in MainActivity for ACTION_SEND_MULTIPLE
- Add shareMultipleContentUris binding in PlatformBridge
- Add _shareSelected and _performBatchConversion methods to DownloadedAlbumScreen and LocalAlbumScreen
- Add batch convert bottom sheet UI with format/bitrate selection (MP3/Opus, 128k-320k)
- Add share & convert action buttons to selection bottom bar in both screens
- Add batch convert with full SAF support: temp copy, write-back, history update
- Add share/convert selection strings to l10n (all supported locales + app_en.arb)
- Add queue tab selection share/convert feature (queue_tab.dart)
- Update donate page
- Update go.sum with bumped dependency hashes

lib/services/platform_bridge.dart

@@ -244,6 +244,14 @@ class PlatformBridge {
     return result as bool? ?? false;
   }
 
+  static Future<bool> shareMultipleContentUris(List<String> uris, {String title = ''}) async {
+    final result = await _channel.invokeMethod('shareMultipleContentUris', {
+      'uris': uris,
+      'title': title,
+    });
+    return result as bool? ?? false;
+  }
+
   static Future<Map<String, dynamic>> fetchLyrics(
     String spotifyId,
     String trackName,