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>

.claude/context/styling.md

@@ -137,6 +137,12 @@ blocks are noted where they matter:
   `lib/widgets/tabs/` (`tab_state_slivers.dart`, `tab_day_groups.dart`,
   `tab_location.dart`) — the loading/error/empty-state slivers and
   day/month grouping helpers shared by Recent/Activity/Trash/Shares.
+  `tab_state_slivers.dart` also has `topBarSliver`, which every regular
+  tab's view uses to plant its own `AppTopBar` instance as the first sliver
+  in its `CustomScrollView` (a thin wrapper around the framework's
+  `SliverFloatingHeader`) instead of the shell passing one shared instance
+  to `Scaffold.appBar` - see its doc comment and `architecture.md`'s
+  "Mobile" bullet for why.
   `lib/widgets/settings/` — Settings' 8 section widgets plus
   `settings_section.dart`'s `SettingsSection`/`showSettingsPicker` and
   `settings_dialogs.dart`'s `confirmRemoveAccount`.