Refactor screens to MVC architecture with modular widgets

- Extract business logic from screens into dedicated controllers
- Split large screen files into smaller, reusable widget components
- Add controllers for AddSubscriptionScreen and DetailScreen
- Create modular widgets for subscription and detail features
- Improve code organization and maintainability
- Remove duplicated code and improve reusability

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

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

lib/widgets/exchange_rate_widget.dart

@@ -137,7 +137,7 @@ class _ExchangeRateWidgetState extends State<ExchangeRateWidget> {
       return const SizedBox.shrink(); // 표시할 필요가 없으면 빈 위젯 반환
     }
 
-    return Column(
+    return const Column(
       crossAxisAlignment: CrossAxisAlignment.end,
       children: [
         // 이 위젯은 이제 환율 정보만 제공하고, 실제 UI는 스크린에서 구성