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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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app.js

@@ -951,3 +951,89 @@ const hashId = location.hash.replace(/^#/, "");
     : (SUTRAS.some((s) => s.id === prefs.sutra) ? prefs.sutra : SUTRAS[0]?.id);
   if (initSutraId) selectSutra(initSutraId);
 })();
+
+// ===== 隐藏后台：2 秒内连点页脚提示 6 次打开管理面板（来源链接点击不计数） =====
+const footHint = document.getElementById("foot-hint");
+let hintClicks = 0;
+let hintTimer = null;
+footHint.addEventListener("click", (e) => {
+  if (e.target.closest("a")) return;
+  hintClicks += 1;
+  clearTimeout(hintTimer);
+  hintTimer = setTimeout(() => { hintClicks = 0; }, 2000);
+  if (hintClicks >= 6) {
+    hintClicks = 0;
+    openAdmin();
+  }
+});
+
+/** 字节数转可读文本。 */
+function fmtBytes(n) {
+  if (n >= 1 << 20) return `${(n / (1 << 20)).toFixed(1)} MB`;
+  if (n >= 1 << 10) return `${(n / (1 << 10)).toFixed(1)} KB`;
+  return `${n} B`;
+}
+
+/** 秒数转可读运行时长（如 2天3时 / 4时12分 / 35秒）。 */
+function fmtUptime(sec) {
+  const d = Math.floor(sec / 86400);
+  const h = Math.floor((sec % 86400) / 3600);
+  const m = Math.floor((sec % 3600) / 60);
+  if (d) return `${d}天${h}时`;
+  if (h) return `${h}时${m}分`;
+  if (m) return `${m}分${sec % 60}秒`;
+  return `${sec}秒`;
+}
+
+/** 打开管理面板：本机偏好/播放状态 + 服务器 /api/stats 统计，可点"刷新"重拉。 */
+function openAdmin() {
+  const mask = document.createElement("div");
+  mask.className = "admin-mask";
+  const card = document.createElement("div");
+  card.className = "admin-card";
+  card.setAttribute("role", "dialog");
+  card.setAttribute("aria-label", "管理面板");
+  const voiceName = voiceSelect.options[voiceSelect.selectedIndex]?.textContent || "-";
+  const bgmName = bgmSelect.options[bgmSelect.selectedIndex]?.textContent || "关";
+  card.innerHTML = `
+    <div class="admin-title">后台</div>
+    <div class="admin-sec">本机</div>
+    <div class="admin-rows">
+      <div class="admin-row"><span>经典</span><b>${sutra ? sutra.name : "-"}</b></div>
+      <div class="admin-row"><span>音色</span><b>${voiceName}</b></div>
+      <div class="admin-row"><span>语速</span><b>${rateSelect.value}×</b></div>
+      <div class="admin-row"><span>配乐</span><b>${bgmName}</b></div>
+      <div class="admin-row"><span>循环 / 夜间</span><b>${loopMode ? "开" : "关"} / ${document.body.classList.contains("night") ? "开" : "关"}</b></div>
+      <div class="admin-row"><span>播放</span><b>${playing ? `第 ${currentSeg + 1}/${segments.length} 段` : "未在播"}</b></div>
+      <div class="admin-row"><span>浏览器</span><b>${/Mobile/.test(navigator.userAgent) ? "移动端" : "桌面端"}${navigator.serviceWorker?.controller ? " · PWA" : ""}</b></div>
+    </div>
+    <div class="admin-sec">服务器</div>
+    <div class="admin-rows" id="admin-server"><div class="admin-row"><span>载入中…</span></div></div>
+    <div class="admin-actions">
+      <button type="button" class="admin-btn" id="admin-refresh">刷新</button>
+      <button type="button" class="admin-btn" id="admin-close">关闭</button>
+    </div>`;
+  mask.appendChild(card);
+  document.body.appendChild(mask);
+  const close = () => mask.remove();
+  mask.addEventListener("click", (e) => { if (e.target === mask) close(); });
+  card.querySelector("#admin-close").addEventListener("click", close);
+
+  const loadStats = async () => {
+    const box = card.querySelector("#admin-server");
+    try {
+      const s = await (await fetch("/api/stats")).json();
+      const hitRate = s.tts_requests ? `${Math.round((s.cache_hits / s.tts_requests) * 100)}%` : "-";
+      box.innerHTML = `
+        <div class="admin-row"><span>运行时长</span><b>${fmtUptime(s.uptime_sec)}</b></div>
+        <div class="admin-row"><span>访客 IP</span><b>${s.visitors}</b></div>
+        <div class="admin-row"><span>TTS 请求</span><b>${s.tts_requests}（命中 ${s.cache_hits} · ${hitRate}）</b></div>
+        <div class="admin-row"><span>语音缓存</span><b>${s.cache_files} 个 · ${fmtBytes(s.cache_bytes)}</b></div>
+        <div class="admin-row"><span>经典数 / 模型</span><b>${s.sutra_count} 部 · ${s.model}</b></div>`;
+    } catch {
+      box.innerHTML = '<div class="admin-row"><span>服务器统计不可用</span></div>';
+    }
+  };
+  loadStats();
+  card.querySelector("#admin-refresh").addEventListener("click", loadStats);
+}