Merge: make the shell top bar hide/reveal with scroll, per tab

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/views/photos_view.dart

@@ -54,7 +54,11 @@ class _MonthGroup {
 class PhotosView extends StatefulWidget {
   final ScrollController scrollController;
 
-  const PhotosView({super.key, required this.scrollController});
+  /// This tab's own shell top bar, planted as its first sliver - see
+  /// `buildAppTabView`'s doc comment. Null on desktop and while picking.
+  final PreferredSizeWidget? topBar;
+
+  const PhotosView({super.key, required this.scrollController, this.topBar});
 
   @override
   State<PhotosView> createState() => _PhotosViewState();
@@ -240,6 +244,7 @@ class _PhotosViewState extends State<PhotosView> {
     );
 
     final List<Widget> contentSlivers = [
+      if (widget.topBar != null) topBarSliver(widget.topBar!),
       // Sticky while browsing; once selecting, the selection bar takes over
       // the same slot instead.
       SliverPersistentHeader(