fix(ui): on a phone, a snack bar stands above the screen's floating button

Below medium windows the frame's scaffold showed the screens' snack bars,
under the floating button of the screen's own scaffold, so a tap on a
snack bar's Undo hit the button instead (found 9 Nov 2026, Todo's Undo
after deleting the completed tasks). The screens get a messenger of their
own there, and their scaffolds lift the snack bars above their buttons, as
Material places them. From medium windows on the frame's scaffold still
shows them, once: a messenger shows a snack bar on each of its scaffolds
not inside another, as two panes side by side are. As the window grows
past a phone's, the phone's snack bars go, since a scaffold whose
messenger changes keeps the one it showed for good.

Whats-New: On a phone, a message's Undo is no longer hidden under the + button.
Whats-New-De: Auf dem Handy liegt das Rückgängig einer Meldung nicht mehr unter der +-Schaltfläche.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/lib/src/widget/navigation_frame.dart

@@ -85,6 +85,15 @@ class const NavigationFrame({
 class _NavigationFrameState() extends State<NavigationFrame> {
   final _tint = ScreenTint();
 
+  /// The screens' own messenger below medium windows: their snack bars
+  /// show on the screens' own scaffolds, which lift them above their
+  /// floating buttons, as Material places them; on the frame's they lay
+  /// under the button, whose taps took the snack bar's Undo (found 9 Nov
+  /// 2026). From medium windows on the frame's scaffold shows them, once:
+  /// a messenger shows its snack bars on each of its scaffolds not inside
+  /// another, as two panes side by side are.
+  final _phoneMessenger = GlobalKey<ScaffoldMessengerState>();
+
   /// The latest wave sent by a tap; none before one.
   NavigationWave? _wave;
 
@@ -109,6 +118,13 @@ class _NavigationFrameState() extends State<NavigationFrame> {
   Widget build(BuildContext context) {
     final destinations = widget.destinations;
     if (context.windowSize.isAtLeastMedium) {
+      // The phone's messenger goes as the window grows, and a scaffold
+      // whose messenger changes would keep the snack bar it showed for
+      // good: they go first. Its scaffolds are below this frame, so they
+      // may be marked to build again while it builds.
+      _phoneMessenger.currentState
+        ?..clearSnackBars()
+        ..removeCurrentSnackBar();
       return ScreenTintScope(
         notifier: _tint,
         child: TintFrame(
@@ -218,7 +234,7 @@ class _NavigationFrameState() extends State<NavigationFrame> {
       // it keeps its size and lifts the field instead (Halkyon Learn's ADR
       // 0021, D69).
       resizeToAvoidBottomInset: false,
-      body: widget.child,
+      body: ScaffoldMessenger(key: _phoneMessenger, child: widget.child),
       // Without the bar, a strip as high as the system's own bar, in the
       // screen's background, keeps every screen inside clear of it.
       bottomNavigationBar: widget.hidesNavigationBar