fix: theme toggle causing auth issues due to auto-dispose

- Add @persistentRiverpod annotation for providers that need keepAlive
- ThemeProvider now persists for app lifetime
- Refactored API clients to use @persistentRiverpod
- Documented in ARCHITECTURE.md

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

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

lib/core/api/api_client.dart

@@ -2,6 +2,7 @@ import 'package:dio/dio.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 import 'package:tatlock_ui/core/api/api_interceptors.dart';
 import 'package:tatlock_ui/core/config/app_config.dart';
+import 'package:tatlock_ui/core/providers/annotations.dart';
 
 import 'api_client_native.dart' if (dart.library.html) 'api_client_web.dart'
     as platform;
@@ -9,10 +10,7 @@ import 'api_client_native.dart' if (dart.library.html) 'api_client_web.dart'
 part 'api_client.g.dart';
 
 /// Provides the Dio instance for Core API.
-///
-/// Uses keepAlive to prevent auto-dispose - the AuthInterceptor stores
-/// a Ref that must remain valid for the lifetime of API requests.
-@Riverpod(keepAlive: true)
+@persistentRiverpod
 Dio coreApiClient(Ref ref) {
   final options = BaseOptions(
     baseUrl: AppConfig.coreApiUrl,
@@ -36,10 +34,7 @@ Dio coreApiClient(Ref ref) {
 }
 
 /// Provides the Dio instance for Tatlock API.
-///
-/// Uses keepAlive to prevent auto-dispose - the AuthInterceptor stores
-/// a Ref that must remain valid for the lifetime of API requests.
-@Riverpod(keepAlive: true)
+@persistentRiverpod
 Dio tatlockApiClient(Ref ref) {
   final options = BaseOptions(
     baseUrl: AppConfig.tatlockApiUrl,