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/screens/category_management_screen.dart

@@ -1,7 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 import '../providers/category_provider.dart';
-import '../models/category_model.dart';
 
 class CategoryManagementScreen extends StatefulWidget {
   const CategoryManagementScreen({super.key});