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/ios/Runner/AppDelegate.swift

@@ -99,6 +99,15 @@ import UserNotifications
       )
     }
 
+    if let navigationGlassRegistrar = engineBridge.pluginRegistry.registrar(
+      forPlugin: "BuzzNavigationGlassButton"
+    ) {
+      navigationGlassRegistrar.register(
+        NavigationGlassButtonFactory(messenger: messenger),
+        withId: "buzz/navigation_glass"
+      )
+    }
+
     if let stickyDateGlassRegistrar = engineBridge.pluginRegistry.registrar(
       forPlugin: "BuzzStickyDateGlassHeader"
     ) {