Make the shell top bar hide/reveal with scroll, per tab

Each regular tab (Files/Photos/Favorites/Shares/Recent/Activity/Trash)
now plants its own AppTopBar as the first sliver in its own
CustomScrollView, via a new topBarSliver() helper (tab_state_slivers.dart)
wrapping the framework's SliverFloatingHeader, instead of one shared
instance living in Scaffold.appBar.

This gives each tab's top bar Material's native floating-app-bar
behavior - tied to that tab's own ScrollController - scrolling away as
the list scrolls down and reappearing the moment the drag reverses,
not only once scrolled all the way back to the top, which a single
fixed Scaffold.appBar (or the "put it in a SliverToBoxAdapter" escape
hatch NooTopBar's doc comment already mentioned) can't do. It sits
above each tab's own pinned in-content sticky header (the sort/filter
row, or the selection bar that replaces it), which keeps working
unchanged underneath it.

main.dart builds a separate, correctly-labelled AppTopBar per tab
(rather than one for "whichever tab is selected") and passes it through
buildAppTabView's new optional topBar parameter; it's null on desktop
(which keeps its own NooToolbar) and while picking, matching the old
Scaffold.appBar: pickRequest == null guard.

SliverFloatingHeader sizes itself from the bar's own natural layout
(like SliverToBoxAdapter) rather than a fixed extent declared up front,
so NooTopBar's own internal SafeArea keeps handling the status-bar inset
correctly with no extra height math needed on this side.

Adds a widget test (test/widgets/tabs/tab_state_slivers_test.dart)
driving a raw gesture mid-drag to verify the reveal-follows-finger
behavior, since a single tester.drag() only pumps after the gesture -
including release - has already finished, which doesn't exercise the
live per-frame reveal this change depends on.

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

lib/widgets/tabs/tab_state_slivers.dart

@@ -94,3 +94,29 @@ const List<Widget> tabBottomInsetSlivers = [
   SliverToBoxAdapter(child: SizedBox(height: 100)),
   SliverFillRemaining(hasScrollBody: false, child: SizedBox()),
 ];
+
+/// Wraps a tab's shell top bar ([AppTopBar], passed in as the generic
+/// [PreferredSizeWidget] it implements - this file can't import
+/// `app_top_bar.dart` without a cycle) as that tab's own first sliver,
+/// living inside its `CustomScrollView` instead of `Scaffold.appBar`. Gives
+/// it Material's native "floating app bar" behavior, via the framework's own
+/// [SliverFloatingHeader]: it scrolls away as the list scrolls down, and -
+/// unlike a plain `SliverToBoxAdapter`, which only reappears once scrolled
+/// all the way back to the top - reappears immediately on any upward
+/// scroll, following the finger while dragging and settling fully open or
+/// fully closed once the gesture ends.
+///
+/// [SliverFloatingHeader] sizes itself from [topBar]'s own natural layout
+/// (like `SliverToBoxAdapter`) rather than a fixed extent declared up
+/// front - so [topBar]'s own internal `SafeArea` (see `NooTopBar`'s doc
+/// comment) already accounts for the status-bar inset correctly, with no
+/// extra height math needed here (unlike building this on the general-
+/// purpose `SliverPersistentHeader` would have required).
+///
+/// Sits above a tab's own pinned in-content header (built with
+/// [StickyHeaderDelegate] - the sort/filter controls row, or the selection
+/// bar that replaces it) - put this sliver first in `contentSlivers` so
+/// that header stays exactly where it already is, independent of whether
+/// [topBar] is currently shown or scrolled away.
+Widget topBarSliver(PreferredSizeWidget topBar) =>
+    SliverFloatingHeader(child: topBar);