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/theme/theme_provider.dart

@@ -4,6 +4,7 @@ import 'package:flutter/material.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'package:tatlock_ui/core/auth/auth_provider.dart';
+import 'package:tatlock_ui/core/providers/annotations.dart';
 
 part 'theme_provider.g.dart';
 
@@ -23,7 +24,7 @@ enum ThemeSetting {
 ///
 /// Syncs with API preferences when user is authenticated. On login, the theme
 /// from API preferences takes precedence over local storage.
-@riverpod
+@persistentRiverpod
 class ThemeNotifier extends _$ThemeNotifier {
   static const _prefsKey = 'theme_setting';