feat: expanded sliver app bar (#19827)

* use mutex

* feat: cool app bar

* animation

* adapt to more pages

* animation

* better animation

* fix: asset count

* Revert "fix: asset count"

This reverts commit 673a5b264b.

* fix: asset count

* fix: shaky animation on Android

* tunning

* offset SizedBox to fix scroll jump on multiselect

---------

Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>

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

@@ -210,6 +210,9 @@ class TimelineService {
   Future<void> preCacheAssets(int index) =>
       _mutex.run(() => _loadAssets(index, math.min(5, _totalAssets - index)));
 
+  BaseAsset getRandomAsset() =>
+      _buffer.elementAt(math.Random().nextInt(_buffer.length));
+
   BaseAsset getAsset(int index) {
     if (!hasRange(index, 1)) {
       throw RangeError(