Unify mobile loading spinners (#3314)

## What
- add the shared desktop-style arc spinner for mobile
- replace app loading indicators with the shared component
- preserve a static pose when reduced motion is enabled

## Stack
- follows #3313

## Validation
- `just mobile-check`
- focused spinner and pairing widget tests

---------

Signed-off-by: kenny lopez <klopez4212@gmail.com>

mobile/lib/app.dart

@@ -16,6 +16,7 @@ import 'shared/auth/auth.dart';
 import 'shared/deeplink/pending_deep_link_provider.dart';
 import 'shared/relay/relay.dart';
 import 'shared/theme/theme.dart';
+import 'shared/widgets/buzz_loading_indicator.dart';
 
 class App extends HookConsumerWidget {
   const App({super.key});
@@ -113,6 +114,10 @@ class _SplashScreen extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return const Scaffold(body: Center(child: CircularProgressIndicator()));
+    return const Scaffold(
+      body: Center(
+        child: BuzzLoadingIndicator(size: 56, semanticLabel: 'Starting Buzz'),
+      ),
+    );
   }
 }