完成产品审查剩余修复：新手指引、反馈入口、付费墙预览

P1-4：新增 WelcomeOverlay 新手指引
- 首次访问时显示 3 步引导：①从地图选城市 ②查看城市简报 ③解锁 Pro 深入分析
- 圆点进度指示、跳过/下一步按钮、点背景可关闭
- 看过一次后 localStorage 标记不再显示

P1-5：付费墙增加功能预览
- HistoryModal 在付费墙上方展示功能说明文案
- 新增 i18n 键 history.previewTitle / history.previewDesc（中英双语）

P2-9：新增反馈入口
- 顶栏增加 Telegram 反馈按钮（MessageCircle 图标）
- 点击跳转 PolyWeather 社群

Tested: npx tsc --noEmit

frontend/lib/i18n.ts

@@ -77,6 +77,8 @@ const MESSAGES: Record<Locale, Record<string, string>> = {
     "history.loading": "正在获取历史数据...",
     "history.error": "获取历史信息失败",
     "history.empty": "近 15 天暂无该城市历史数据",
+    "history.previewTitle": "历史准确率对账",
+    "history.previewDesc": "对比 DEB 预报与实际结算温度，查看命中率、MAE 和模型对比。升级 Pro 即可解锁。",
     "history.hitRate": "DEB 结算胜率 (METAR)",
     "history.mae": "DEB MAE",
     "history.debHitRate": "DEB 结算胜率 (METAR)",
@@ -274,6 +276,8 @@ const MESSAGES: Record<Locale, Record<string, string>> = {
     "history.loading": "Loading historical data...",
     "history.error": "Failed to load historical data",
     "history.empty": "No historical records for this city in the last 15 days",
+    "history.previewTitle": "Historical Reconciliation",
+    "history.previewDesc": "Compare DEB forecasts to actual settlement temperatures with hit rate, MAE, and model comparison. Unlock Pro to access.",
     "history.hitRate": "DEB Settlement Hit Rate (METAR)",
     "history.mae": "DEB MAE",
     "history.debHitRate": "DEB Settlement Hit Rate (METAR)",