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);