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/common/form_fields/currency_input_field.dart

@@ -0,0 +1,138 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:intl/intl.dart';
+import 'base_text_field.dart';
+
+/// 통화 입력 필드 위젯
+/// 원화(KRW)와 달러(USD)를 지원하며 자동 포맷팅을 제공합니다.
+class CurrencyInputField extends StatefulWidget {
+  final TextEditingController controller;
+  final String currency; // 'KRW' or 'USD'
+  final String? label;
+  final String? hintText;
+  final Function(double?)? onChanged;
+  final String? Function(String?)? validator;
+  final FocusNode? focusNode;
+  final TextInputAction? textInputAction;
+  final Function()? onEditingComplete;
+  final bool enabled;
+
+  const CurrencyInputField({
+    super.key,
+    required this.controller,
+    required this.currency,
+    this.label,
+    this.hintText,
+    this.onChanged,
+    this.validator,
+    this.focusNode,
+    this.textInputAction,
+    this.onEditingComplete,
+    this.enabled = true,
+  });
+
+  @override
+  State<CurrencyInputField> createState() => _CurrencyInputFieldState();
+}
+
+class _CurrencyInputFieldState extends State<CurrencyInputField> {
+  late TextEditingController _formattedController;
+
+  @override
+  void initState() {
+    super.initState();
+    _formattedController = TextEditingController();
+    _updateFormattedValue();
+    widget.controller.addListener(_onControllerChanged);
+  }
+
+  @override
+  void dispose() {
+    widget.controller.removeListener(_onControllerChanged);
+    _formattedController.dispose();
+    super.dispose();
+  }
+
+  void _onControllerChanged() {
+    _updateFormattedValue();
+  }
+
+  void _updateFormattedValue() {
+    final value = double.tryParse(widget.controller.text.replaceAll(',', ''));
+    if (value != null) {
+      _formattedController.text = _formatCurrency(value);
+    } else {
+      _formattedController.text = '';
+    }
+  }
+
+  String _formatCurrency(double value) {
+    if (widget.currency == 'KRW') {
+      return NumberFormat.decimalPattern().format(value.toInt());
+    } else {
+      return NumberFormat('#,##0.00').format(value);
+    }
+  }
+
+  double? _parseValue(String text) {
+    final cleanText = text.replaceAll(',', '').replaceAll('₩', '').replaceAll('\$', '').trim();
+    return double.tryParse(cleanText);
+  }
+
+  String get _prefixText {
+    return widget.currency == 'KRW' ? '₩ ' : '\$ ';
+  }
+
+  String get _defaultHintText {
+    return widget.currency == 'KRW' ? '금액을 입력하세요' : 'Enter amount';
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return BaseTextField(
+      controller: _formattedController,
+      focusNode: widget.focusNode,
+      label: widget.label,
+      hintText: widget.hintText ?? _defaultHintText,
+      textInputAction: widget.textInputAction,
+      keyboardType: const TextInputType.numberWithOptions(decimal: true),
+      inputFormatters: [
+        FilteringTextInputFormatter.allow(RegExp(r'[\d,.]')),
+      ],
+      prefixText: _prefixText,
+      onEditingComplete: widget.onEditingComplete,
+      enabled: widget.enabled,
+      onChanged: (value) {
+        final parsedValue = _parseValue(value);
+        if (parsedValue != null) {
+          widget.controller.text = parsedValue.toString();
+          widget.onChanged?.call(parsedValue);
+        } else {
+          widget.controller.text = '';
+          widget.onChanged?.call(null);
+        }
+        
+        // 포맷팅 업데이트
+        if (parsedValue != null) {
+          final formattedText = _formatCurrency(parsedValue);
+          if (formattedText != value) {
+            _formattedController.value = TextEditingValue(
+              text: formattedText,
+              selection: TextSelection.collapsed(offset: formattedText.length),
+            );
+          }
+        }
+      },
+      validator: widget.validator ?? (value) {
+        if (value == null || value.isEmpty) {
+          return '금액을 입력해주세요';
+        }
+        final parsedValue = _parseValue(value);
+        if (parsedValue == null || parsedValue <= 0) {
+          return '올바른 금액을 입력해주세요';
+        }
+        return null;
+      },
+    );
+  }
+}