feat: i18n 中英双语界面 — i18next + react-i18next

- 安装 i18next / react-i18next / i18next-browser-languagedetector
- 新建 src/i18n.ts 配置（fallback zh）
- 中/英翻译文件各 ~50 条目
- App.tsx 新增 EN/中 语言切换按钮
- ExportPanel + QrPreview + ModePanel + HistoryList + ErrorBoundary
- 全部 7 种模式组件均支持双语
- 12 前端测试通过，tsc 零错误

gui/src-frontend/public/locales/zh/translation.json

@@ -0,0 +1,80 @@
+{
+  "app": {
+    "title": "QRGen",
+    "encodingModes": "编码模式",
+    "exportOptions": "导出选项",
+    "decode": "解码"
+  },
+  "mode": {
+    "text": "文本",
+    "url": "URL",
+    "wifi": "WiFi",
+    "vcard": "vCard",
+    "email": "Email",
+    "phone": "电话",
+    "sms": "SMS"
+  },
+  "export": {
+    "eccLevel": "纠错级别",
+    "moduleSize": "模块大小",
+    "margin": "边距",
+    "copySvg": "复制 SVG",
+    "exportPng": "导出 PNG",
+    "exportSvg": "导出 SVG",
+    "exporting": "导出中...",
+    "selectImage": "选择图片解码",
+    "decoding": "解码中..."
+  },
+  "preview": {
+    "loading": "生成中...",
+    "empty": "输入内容生成 QR 码",
+    "version": "版本",
+    "mask": "掩码"
+  },
+  "history": {
+    "title": "历史记录",
+    "clear": "清空",
+    "empty": "暂无记录"
+  },
+  "wifi": {
+    "ssid": "SSID",
+    "password": "密码",
+    "none": "无密码",
+    "hidden": "隐藏"
+  },
+  "vcard": {
+    "name": "姓名",
+    "phone": "电话",
+    "email": "邮箱",
+    "company": "公司",
+    "address": "地址"
+  },
+  "email": {
+    "to": "收件人",
+    "subject": "主题",
+    "body": "正文"
+  },
+  "phone": {
+    "placeholder": "输入电话号码"
+  },
+  "sms": {
+    "number": "电话号码",
+    "message": "短信内容"
+  },
+  "text": {
+    "placeholder": "输入任意文本..."
+  },
+  "error": {
+    "appError": "应用发生错误",
+    "reload": "重新加载",
+    "decodeFailed": "解码失败",
+    "exportPngFailed": "导出 PNG 失败",
+    "exportSvgFailed": "导出 SVG 失败",
+    "copyFailed": "复制失败"
+  },
+  "dialog": {
+    "imageFiles": "图片文件",
+    "pngImage": "PNG 图片",
+    "svgImage": "SVG 图片"
+  }
+}