Fix mobile thread tail and iOS channel header (#6399)

## Summary

- Hide the thread Latest control once the lazy tail is fully reached.
- Polish the mobile channel header and use native iOS liquid glass for
Back.
- Keep the two-line header aligned and safe at larger text sizes.

## Validation

- `just mobile-check`
- `flutter test` (1,553 tests)
- Signed iPhone build installed and launched

---------

Signed-off-by: kenny lopez <klopez4212@gmail.com>
Co-authored-by: Carl <3c4caeafb646d23867f1c4832e68211d77e2561946171625f75c3ce1a3f2670f@buzz.block.builderlab.xyz>

mobile/test/shared/widgets/frosted_app_bar_test.dart

@@ -1,6 +1,9 @@
 import 'package:buzz/shared/theme/theme.dart';
 import 'package:buzz/shared/widgets/frosted_app_bar.dart';
+import 'package:buzz/shared/widgets/ios_glass_navigation_button.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -83,4 +86,113 @@ void main() {
     expect(reportedHeight, closeTo(titleContentHeight + Grid.xs + 1, 0.01));
     expect(tester.takeException(), isNull);
   });
+
+  testWidgets('uses the native glass back control on iOS', (tester) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    addTearDown(() => debugDefaultTargetPlatformOverride = null);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: AppTheme.light(),
+        home: Builder(
+          builder: (context) => TextButton(
+            onPressed: () => Navigator.of(context).push(
+              MaterialPageRoute<void>(
+                builder: (_) => const Stack(
+                  children: [FrostedAppBar(title: Text('Destination'))],
+                ),
+              ),
+            ),
+            child: const Text('Open'),
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('Open'));
+    await tester.pumpAndSettle();
+
+    final nativeBack = tester.widget<UiKitView>(find.byType(UiKitView));
+    expect(nativeBack.viewType, 'buzz/navigation_glass');
+    expect(nativeBack.creationParams, containsPair('icon', 'back'));
+    expect(
+      nativeBack.creationParams,
+      containsPair('accessibilityLabel', 'Back'),
+    );
+    expect(nativeBack.creationParams, containsPair('hitTargetWidth', 48.0));
+    expect(nativeBack.creationParams, containsPair('hitTargetHeight', 48.0));
+    expect(find.byTooltip('Back'), findsOneWidget);
+    expect(
+      tester.widget<Tooltip>(find.byTooltip('Back')).excludeFromSemantics,
+      isTrue,
+    );
+    debugDefaultTargetPlatformOverride = null;
+  });
+
+  testWidgets('replaces native glass while a Flutter backdrop is active', (
+    tester,
+  ) async {
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    addTearDown(() => debugDefaultTargetPlatformOverride = null);
+    final suppressNativeView = ValueNotifier(false);
+    addTearDown(suppressNativeView.dispose);
+    var pressCount = 0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: AppTheme.light(),
+        home: Scaffold(
+          body: IosGlassNavigationButton(
+            icon: IosGlassNavigationIcon.back,
+            semanticLabel: 'Back',
+            onPressed: () => pressCount++,
+            nativeViewSuppressed: suppressNativeView,
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(UiKitView), findsOneWidget);
+    expect(
+      find.byKey(const ValueKey('ios-glass-navigation-flutter-fallback')),
+      findsNothing,
+    );
+    expect(find.bySemanticsLabel('Back'), findsNothing);
+
+    suppressNativeView.value = true;
+    await tester.pump();
+
+    expect(find.byType(UiKitView), findsNothing);
+    expect(
+      find.byKey(const ValueKey('ios-glass-navigation-flutter-fallback')),
+      findsOneWidget,
+    );
+    final fallbackFinder = find.bySemanticsLabel('Back');
+    expect(fallbackFinder, findsOneWidget);
+    final fallbackSemantics = tester.getSemantics(fallbackFinder);
+    expect(fallbackSemantics.flagsCollection.isButton, isTrue);
+    expect(
+      fallbackSemantics.flagsCollection.isEnabled.toString(),
+      'Tristate.isTrue',
+    );
+    expect(
+      fallbackSemantics.getSemanticsData().hasAction(SemanticsAction.tap),
+      isTrue,
+    );
+    tester.binding.performSemanticsAction(
+      SemanticsActionEvent(
+        type: SemanticsAction.tap,
+        viewId: tester.view.viewId,
+        nodeId: fallbackSemantics.id,
+      ),
+    );
+    await tester.pump();
+    expect(pressCount, 1);
+
+    suppressNativeView.value = false;
+    await tester.pump();
+
+    expect(find.byType(UiKitView), findsOneWidget);
+    debugDefaultTargetPlatformOverride = null;
+  });
 }

mobile/test/shared/widgets/modal_presentation_test.dart

@@ -111,7 +111,13 @@ void main() {
       await tester.tap(find.text('Open sheet'));
       await tester.pumpAndSettle();
 
-      final nativeSurface = tester.widget<UiKitView>(find.byType(UiKitView));
+      final nativeSurface = tester.widget<UiKitView>(
+        find.byWidgetPredicate(
+          (widget) =>
+              widget is UiKitView &&
+              widget.viewType == 'buzz/concentric_sheet_surface',
+        ),
+      );
       expect(
         nativeSurface.creationParams,
         containsPair('color', lightColorScheme.surface.toARGB32()),
@@ -131,6 +137,13 @@ void main() {
       expect(contentClip.borderRadius, BorderRadius.circular(Radii.dialog * 2));
       expect(contentClip.clipBehavior, Clip.antiAlias);
       expect(find.text('Members'), findsOneWidget);
+      final nativeClose = tester.widget<UiKitView>(
+        find.byWidgetPredicate(
+          (widget) =>
+              widget is UiKitView && widget.viewType == 'buzz/navigation_glass',
+        ),
+      );
+      expect(nativeClose.creationParams, containsPair('icon', 'close'));
     } finally {
       tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(
         surfaceChannel,