fix(auth): prevent AuthNotifier auto-dispose causing theme toggle logout

Applied @persistentRiverpod annotation to AuthNotifier so it persists
for app lifetime. Previously, theme changes could trigger AuthProvider
rebuild via auto-dispose, causing AsyncLoading state and auth issues.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/core/auth/auth_provider.dart

@@ -6,6 +6,7 @@ import 'package:riverpod_annotation/riverpod_annotation.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import '../config/app_config.dart';
+import '../providers/annotations.dart';
 import 'auth_datasource.dart';
 import 'auth_state.dart';
 import 'oidc_service.dart';
@@ -24,7 +25,7 @@ part 'auth_provider.g.dart';
 ///
 /// After OIDC authentication, syncs with core-api via POST /auth/sync
 /// to get user profile, roles, and preferences.
-@riverpod
+@persistentRiverpod
 class AuthNotifier extends _$AuthNotifier {
   // Storage keys
   static const _accessTokenKey = 'auth_access_token';