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>

docs/ARCHITECTURE.md

@@ -420,6 +420,41 @@ ContainerRepository containerRepository(Ref ref) {
 }
 ```
 
+### Persistent Providers
+
+By default, `@riverpod` generates providers with `isAutoDispose: true`, meaning they dispose when no longer watched. This causes issues for:
+
+- **API clients** with interceptors that store a `Ref`
+- **App-level state** like theme, auth, config
+- **Providers with listeners** to other providers
+
+Use `@persistentRiverpod` from `core/providers/annotations.dart` for these cases:
+
+```dart
+import 'package:tatlock_ui/core/providers/annotations.dart';
+
+// ✅ Correct - persists for app lifetime
+@persistentRiverpod
+Dio coreApiClient(Ref ref) { ... }
+
+@persistentRiverpod
+class ThemeNotifier extends _$ThemeNotifier { ... }
+
+// ❌ Wrong - auto-dispose can invalidate stored Ref
+@riverpod
+Dio coreApiClient(Ref ref) { ... }
+```
+
+**When to use `@persistentRiverpod`:**
+
+| Use Case | Annotation |
+|----------|------------|
+| API clients with interceptors | `@persistentRiverpod` |
+| Theme/config providers | `@persistentRiverpod` |
+| Auth state provider | `@persistentRiverpod` |
+| Feature data providers | `@riverpod` (default) |
+| UI state providers | `@riverpod` (default) |
+
 ## File Naming Conventions
 
 | Type | Convention | Example |