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/providers/annotations.dart

@@ -0,0 +1,15 @@
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+
+/// Riverpod annotation for providers that should persist for the app lifetime.
+///
+/// Use this instead of `@riverpod` when:
+/// - The provider holds app-level state (theme, auth, config)
+/// - The provider stores a Ref that must remain valid (API clients with interceptors)
+/// - Disposing would cause flickering or re-initialization issues
+///
+/// Example:
+/// ```dart
+/// @persistentRiverpod
+/// class ThemeNotifier extends _$ThemeNotifier { ... }
+/// ```
+const persistentRiverpod = Riverpod(keepAlive: true);