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/spring_animation_widget.dart

@@ -1,6 +1,5 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/physics.dart';
-import 'dart:math' as math;
 
 /// 물리 기반 스프링 애니메이션을 적용하는 위젯
 class SpringAnimationWidget extends StatefulWidget {
@@ -44,14 +43,6 @@ class _SpringAnimationWidgetState extends State<SpringAnimationWidget>
       duration: const Duration(seconds: 2),
     );
     
-    // 스프링 시뮬레이션
-    final simulation = SpringSimulation(
-      widget.spring,
-      0.0,
-      1.0,
-      0.0,
-    );
-    
     // 오프셋 애니메이션
     _offsetAnimation = Tween<Offset>(
       begin: widget.initialOffset ?? const Offset(0, 50),