feat: 隐藏后台面板 + /api/stats 运行统计

- 连点页脚提示 6 次唤出管理面板：本机偏好/播放状态 + 服务器统计
- server.py 新增 GET /api/stats：运行时长、访客 IP（X-Forwarded-For
  末位取真实 IP 防伪造）、TTS 请求与缓存命中率、缓存体积、经典数
- 计数为进程内存统计，重启清零

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sw.js

@@ -4,7 +4,7 @@
  * - /api/tts 不拦截，直接透传网络
  */
 const CACHE = "fj-shell-v1";
-const SHELL = ["/", "/index.html", "/style.css?v=10", "/app.js?v=12", "/icon.svg", "/icon-180.png", "/icon-192.png", "/icon-512.png", "/manifest.json"];
+const SHELL = ["/", "/index.html", "/style.css?v=11", "/app.js?v=13", "/icon.svg", "/icon-180.png", "/icon-192.png", "/icon-512.png", "/manifest.json"];
 
 self.addEventListener("install", (e) => {
   e.waitUntil(caches.open(CACHE).then((c) => c.addAll(SHELL)).then(() => self.skipWaiting()));