fix(flutter): two test environment issues from CI

smoke_test was hanging in pumpAndSettle because real
flutter_secure_storage MethodChannel calls don't resolve in widget
tests (no platform channel registered). Override secureStorageProvider
with a mocktail FlutterSecureStorage that returns null for every read,
so the GoRouter redirect resolves and ServerUrlScreen renders.

player_provider_test second case was crashing because constructing
MinstrelAudioHandler() instantiates just_audio's AudioPlayer, which
calls setMethodCallHandler during construction — that asserts unless
the WidgetsFlutterBinding is initialized. Add
TestWidgetsFlutterBinding.ensureInitialized() at the top of main().

Both are unit-test plumbing fixes; no production code change.

flutter_client/test/player/player_provider_test.dart

@@ -5,6 +5,12 @@ import 'package:minstrel/player/audio_handler.dart';
 import 'package:minstrel/player/player_provider.dart';
 
 void main() {
+  // MinstrelAudioHandler() instantiates just_audio's AudioPlayer, which calls
+  // setMethodCallHandler on a platform channel during construction. Without an
+  // initialized binding the call asserts. We're not exercising real audio in
+  // the test — this just gives us a working binary messenger.
+  TestWidgetsFlutterBinding.ensureInitialized();
+
   test('audioHandlerProvider must be overridden — bare read throws', () {
     final container = ProviderContainer();
     addTearDown(container.dispose);

flutter_client/test/smoke_test.dart

@@ -1,11 +1,32 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:mocktail/mocktail.dart';
 
 import 'package:minstrel/app.dart';
+import 'package:minstrel/auth/auth_provider.dart';
+
+class _NullStorage extends Mock implements FlutterSecureStorage {}
 
 void main() {
+  setUpAll(() {
+    registerFallbackValue('');
+  });
+
   testWidgets('cold launch lands on server-url screen when no URL stored', (tester) async {
-    await tester.pumpWidget(const ProviderScope(child: MinstrelApp()));
+    // Override secureStorageProvider so all reads return null deterministically.
+    // Without this the real flutter_secure_storage MethodChannel hangs in
+    // widget tests (no platform channel), so the GoRouter redirect never
+    // resolves and ServerUrlScreen never renders.
+    final storage = _NullStorage();
+    when(() => storage.read(key: any(named: 'key'))).thenAnswer((_) async => null);
+
+    await tester.pumpWidget(ProviderScope(
+      overrides: [
+        secureStorageProvider.overrideWithValue(storage),
+      ],
+      child: const MinstrelApp(),
+    ));
     await tester.pumpAndSettle();
     expect(find.text('Connect to your Minstrel'), findsOneWidget);
   });