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/services/exchange_rate_service.dart

@@ -1,6 +1,5 @@
 import 'package:http/http.dart' as http;
 import 'dart:convert';
-import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
 /// 환율 정보 서비스 클래스