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/add_subscription/add_subscription_header.dart

@@ -0,0 +1,88 @@
+import 'package:flutter/material.dart';
+import '../../controllers/add_subscription_controller.dart';
+
+/// 구독 추가 화면의 헤더 섹션
+class AddSubscriptionHeader extends StatelessWidget {
+  final AddSubscriptionController controller;
+  final Animation<double> fadeAnimation;
+  final Animation<Offset> slideAnimation;
+
+  const AddSubscriptionHeader({
+    super.key,
+    required this.controller,
+    required this.fadeAnimation,
+    required this.slideAnimation,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return FadeTransition(
+      opacity: fadeAnimation,
+      child: SlideTransition(
+        position: slideAnimation,
+        child: Container(
+          margin: const EdgeInsets.only(bottom: 24),
+          padding: const EdgeInsets.all(24),
+          decoration: BoxDecoration(
+            borderRadius: BorderRadius.circular(24),
+            gradient: LinearGradient(
+              colors: controller.gradientColors,
+              begin: Alignment.topLeft,
+              end: Alignment.bottomRight,
+            ),
+            boxShadow: [
+              BoxShadow(
+                color: controller.gradientColors[0].withOpacity(0.3),
+                blurRadius: 20,
+                spreadRadius: 0,
+                offset: const Offset(0, 8),
+              ),
+            ],
+          ),
+          child: Row(
+            children: [
+              Container(
+                padding: const EdgeInsets.all(16),
+                decoration: BoxDecoration(
+                  color: Colors.white.withOpacity(0.2),
+                  borderRadius: BorderRadius.circular(16),
+                ),
+                child: const Icon(
+                  Icons.add_rounded,
+                  size: 32,
+                  color: Colors.white,
+                ),
+              ),
+              const SizedBox(width: 16),
+              const Expanded(
+                child: Column(
+                  crossAxisAlignment: CrossAxisAlignment.start,
+                  children: [
+                    Text(
+                      '새 구독 추가',
+                      style: TextStyle(
+                        fontSize: 24,
+                        fontWeight: FontWeight.w800,
+                        color: Colors.white,
+                        letterSpacing: -0.5,
+                      ),
+                    ),
+                    SizedBox(height: 4),
+                    Text(
+                      '서비스 정보를 입력해주세요',
+                      style: TextStyle(
+                        fontSize: 14,
+                        fontWeight: FontWeight.w500,
+                        color: Colors.white70,
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}