fix(mobile): reset asset index on timeline refresh (#25729)

The current asset changes when the timeline refreshes, which can be
quite jarring. Assets are tracked by their index, and that index becomes
stale when the timeline refreshes. This can be resolved by updating the
index of asset based on a unique identifier (like the hero tag).

mobile/lib/domain/services/timeline.service.dart

@@ -227,6 +227,13 @@ class TimelineService {
     return _buffer.elementAt(index - _bufferOffset);
   }
 
+  /// Finds the index of an asset by its heroTag within the current buffer.
+  /// Returns null if the asset is not found in the buffer.
+  int? getIndex(String heroTag) {
+    final index = _buffer.indexWhere((a) => a.heroTag == heroTag);
+    return index >= 0 ? _bufferOffset + index : null;
+  }
+
   Future<void> dispose() async {
     await _bucketSubscription?.cancel();
     _bucketSubscription = null;