feat(server+web): 客户端下载闭环——pangolin-server /downloads + 官网下载链接

- server: /downloads/* 公开静态服务(NewDownloadsHandler,DOWNLOADS_DIR 默认
  /var/lib/pangolin/downloads),防目录穿越/不列目录/缺目录不崩,Cache-Control:
  no-cache 保证 CI 覆盖后永远最新。挂在 /healthz 同级(免鉴权)。含 4 项测试。
- deploy.sh: install -d 下载目录 + server.env 注入 DOWNLOADS_DIR。
- web: site.ts 增 downloads.{android,windows}(→ https://api.yanmeiai.com/downloads/*);
  Download.astro 接上 Android/Windows 按钮,iOS/macOS/Linux 改「即将推出」禁用态。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

web/website/src/i18n/strings.ts

@@ -128,6 +128,7 @@ export const STRINGS: Record<string, [string, string]> = {
   'dl.h': ['全平台，随处可用', 'Every platform, everywhere'],
   'dl.sub': ['一个账户，所有设备同步。下载即用，无需配置。', 'One account syncs every device. Download and go.'],
   'dl.get': ['下载', 'Download'],
+  'dl.soon': ['敬请期待', 'Coming soon'],
 
   'docs.eyebrow': ['文档', 'Docs'],
   'docs.h': ['需要帮助？都在这里', 'Need help? It’s all here'],