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/detail/detail_action_buttons.dart

@@ -0,0 +1,46 @@
+import 'package:flutter/material.dart';
+import '../../controllers/detail_screen_controller.dart';
+import '../common/buttons/primary_button.dart';
+
+/// 상세 화면 액션 버튼 섹션
+/// 저장 버튼을 포함하는 섹션입니다.
+class DetailActionButtons extends StatelessWidget {
+  final DetailScreenController controller;
+  final Animation<double> fadeAnimation;
+  final Animation<Offset> slideAnimation;
+
+  const DetailActionButtons({
+    super.key,
+    required this.controller,
+    required this.fadeAnimation,
+    required this.slideAnimation,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final baseColor = controller.getCardColor();
+
+    return FadeTransition(
+      opacity: fadeAnimation,
+      child: SlideTransition(
+        position: Tween<Offset>(
+          begin: const Offset(0.0, 0.8),
+          end: Offset.zero,
+        ).animate(CurvedAnimation(
+          parent: controller.animationController!,
+          curve: const Interval(0.4, 1.0, curve: Curves.easeOutCubic),
+        )),
+        child: Padding(
+          padding: const EdgeInsets.only(bottom: 80),
+          child: PrimaryButton(
+            text: '변경사항 저장',
+            icon: Icons.save_rounded,
+            onPressed: controller.updateSubscription,
+            isLoading: controller.isLoading,
+            backgroundColor: baseColor,
+          ),
+        ),
+      ),
+    );
+  }
+}