feat: user frontend i18n (zh-CN/en)

- vue-i18n@9 with locale files for zh-CN and en
- Language switcher in topbar
- Navigation, breadcrumb, credits page translated
- Discovery page i18n keys prepared
- Language persisted in localStorage
- Build verified

user-frontend/src/locales/zh-CN.json

@@ -0,0 +1,79 @@
+{
+  "nav": {
+    "discovery": "发现客户",
+    "workspace": "工作台",
+    "customers": "客户管理",
+    "products": "产品库",
+    "quotations": "报价单",
+    "translate": "智能翻译",
+    "marketing": "营销素材",
+    "followup": "跟进提醒",
+    "analytics": "数据分析",
+    "team": "团队协作",
+    "notifications": "通知中心",
+    "profile": "个人中心",
+    "credits": "购买次数",
+    "settings": "设置"
+  },
+  "topbar": {
+    "credits": "次",
+    "purchase": "购买次数",
+    "notifications": "通知"
+  },
+  "discovery": {
+    "title": "发现新客户",
+    "subtitle": "输入产品关键词，找到全球潜在买家",
+    "product_placeholder": "例如：LED lighting, solar panels, auto parts",
+    "market_placeholder": "目标市场（如 US, Germany, 留空为全球）",
+    "search": "搜索买家",
+    "searching": "正在搜索...",
+    "results": "搜索结果",
+    "no_results": "暂无结果，请尝试其他关键词",
+    "credits_cost": "每次搜索消耗 10 次",
+    "analyze": "深度分析",
+    "outreach": "生成开发信",
+    "add_customer": "添加为客户",
+    "visit_website": "访问网站",
+    "match_score": "匹配度",
+    "contact": "联系方式"
+  },
+  "credits": {
+    "title": "信用次数",
+    "balance": "当前余额",
+    "packages": "购买次数",
+    "subscription": "订阅套餐",
+    "history": "消费记录",
+    "purchase": "立即购买",
+    "subscribe": "开通订阅",
+    "per_month": "次/月",
+    "per_credit": "次均",
+    "pay_alipay": "支付宝",
+    "pay_wechat": "微信支付",
+    "confirm_pay": "确认支付",
+    "scan_pay": "扫码支付",
+    "insufficient": "次数不足",
+    "buy_more": "去购买",
+    "total_purchased": "已购买",
+    "total_used": "已使用"
+  },
+  "common": {
+    "loading": "加载中...",
+    "error": "出错了",
+    "retry": "重试",
+    "cancel": "取消",
+    "confirm": "确认",
+    "save": "保存",
+    "delete": "删除",
+    "edit": "编辑",
+    "back": "返回",
+    "no_data": "暂无数据",
+    "search": "搜索",
+    "all": "全部",
+    "status": "状态",
+    "time": "时间"
+  },
+  "lang": {
+    "switch_to": "English",
+    "current": "中文"
+  }
+}