feat: Implement invoice CSV export, add customer title field, and refine invoice item properties and copy method.

lib/models/customer_model.dart

@@ -4,6 +4,7 @@ class Customer {
   final String id;
   final String displayName; // 表示用（電話帳名など）
   final String formalName;  // 請求書用正式名称
+  final String title;       // 敬称（様、殿など）
   final String? department; // 部署名
   final String? address;    // 住所
 
@@ -11,21 +12,24 @@ class Customer {
     required this.id,
     required this.displayName,
     required this.formalName,
+    this.title = "様",
     this.department,
     this.address,
   });
 
   String get invoiceName {
+    String name = formalName;
     if (department != null && department!.isNotEmpty) {
-      return "$formalName\n$department";
+      name = "$formalName\n$department";
     }
-    return formalName;
+    return "$name $title";
   }
 
   Customer copyWith({
     String? id,
     String? displayName,
     String? formalName,
+    String? title,
     String? department,
     String? address,
   }) {
@@ -33,6 +37,7 @@ class Customer {
       id: id ?? this.id,
       displayName: displayName ?? this.displayName,
       formalName: formalName ?? this.formalName,
+      title: title ?? this.title,
       department: department ?? this.department,
       address: address ?? this.address,
     );

lib/models/invoice_models.dart

@@ -2,9 +2,9 @@ import 'customer_model.dart';
 import 'package:intl/intl.dart';
 
 class InvoiceItem {
-  final String description;
-  final num quantity;
-  final int unitPrice;
+  String description;
+  int quantity;
+  int unitPrice;
 
   InvoiceItem({
     required this.description,
@@ -12,7 +12,7 @@ class InvoiceItem {
     required this.unitPrice,
   });
 
-  int get subtotal => (quantity * unitPrice).floor();
+  int get subtotal => quantity * unitPrice;
 }
 
 class Invoice {
@@ -32,12 +32,30 @@ class Invoice {
     this.filePath,
   }) : id = id ?? DateTime.now().millisecondsSinceEpoch.toString();
 
-  String get invoiceNumber => "INV-${DateFormat('yyyyMMdd').format(date)}-${id.substring(id.length - 4)}";
+  String get invoiceNumber => "INV-${DateFormat('yyyyMMdd').format(date)}-${id.substring(id.length > 4 ? id.length - 4 : 0)}";
 
   int get subtotal => items.fold(0, (sum, item) => sum + item.subtotal);
   int get tax => (subtotal * 0.1).floor();
   int get totalAmount => subtotal + tax;
 
+  String toCsv() {
+    final dateFormatter = DateFormat('yyyy/MM/dd');
+    final amountFormatter = NumberFormat("###");
+    
+    StringBuffer buffer = StringBuffer();
+    // ヘッダー (例)
+    buffer.writeln("日付,請求番号,取引先,合計金額,備考");
+    buffer.writeln("${dateFormatter.format(date)},$invoiceNumber,${customer.formalName},$totalAmount,${notes ?? ""}");
+    buffer.writeln("");
+    buffer.writeln("品名,数量,単価,小計");
+    
+    for (var item in items) {
+      buffer.writeln("${item.description},${item.quantity},${item.unitPrice},${item.subtotal}");
+    }
+    
+    return buffer.toString();
+  }
+
   Invoice copyWith({
     String? id,
     Customer? customer,
@@ -50,7 +68,7 @@ class Invoice {
       id: id ?? this.id,
       customer: customer ?? this.customer,
       date: date ?? this.date,
-      items: items ?? this.items,
+      items: items ?? List.from(this.items), // コピーを作成
       notes: notes ?? this.notes,
       filePath: filePath ?? this.filePath,
     );

lib/screens/product_picker_modal.dart

@@ -0,0 +1,62 @@
+import 'package:flutter/material.dart';
+import '../models/invoice_models.dart';
+
+/// 商品マスターから項目を選択するためのモーダル（スタブ実装）
+class ProductPickerModal extends StatefulWidget {
+  final Function(InvoiceItem) onItemSelected;
+
+  const ProductPickerModal({Key? key, required this.onItemSelected}) : super(key: key);
+
+  @override
+  State<ProductPickerModal> createState() => _ProductPickerModalState();
+}
+
+class _ProductPickerModalState extends State<ProductPickerModal> {
+  // 本来はデータベースから取得しますが、現時点ではスタブデータを表示します
+  final List<InvoiceItem> _masterProducts = [
+    InvoiceItem(description: "技術料", quantity: 1, unitPrice: 50000),
+    InvoiceItem(description: "部品代 A", quantity: 1, unitPrice: 15000),
+    InvoiceItem(description: "部品代 B", quantity: 1, unitPrice: 3000),
+    InvoiceItem(description: "出張費", quantity: 1, unitPrice: 10000),
+    InvoiceItem(description: "諸経費", quantity: 1, unitPrice: 5000),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      decoration: const BoxDecoration(
+        color: Colors.white,
+        borderRadius: BorderRadius.vertical(top: Radius.circular(20)),
+      ),
+      child: Column(
+        children: [
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: Row(
+              mainAxisAlignment: MainAxisAlignment.spaceBetween,
+              children: [
+                const Text("商品・サービス選択", style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold)),
+                IconButton(icon: const Icon(Icons.close), onPressed: () => Navigator.pop(context)),
+              ],
+            ),
+          ),
+          const Divider(),
+          Expanded(
+            child: ListView.builder(
+              itemCount: _masterProducts.length,
+              itemBuilder: (context, index) {
+                final product = _masterProducts[index];
+                return ListTile(
+                  leading: const Icon(Icons.inventory_2_outlined),
+                  title: Text(product.description),
+                  subtitle: Text("単価: ￥${product.unitPrice}"),
+                  onTap: () => widget.onItemSelected(product),
+                );
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}