feat: 双层并发 + 全局限流 + 整轮扫尾 + 画廊目录清单

原实现只能「多画廊并行，每个画廊串行下」，单张大画廊下得很慢。
按用户多年使用习惯（5 画廊 × 每画廊 8 张）改造为双层并发。

- 双层并发：画廊并发(默认5) × 页并发(默认8) = 最多 40 张同时下载，
  两个值都在设置面板可调（1-20）。页级 worker 池 + mutex 保护计数。
- 全局限流：对齐参考实现的 nextRequestTime 算法，限所有请求
  （含 /s/ 解析与图片本体），默认 0 秒关闭。
- 整轮扫尾：跑完后自动重扫失败页 1 轮（同样走完整 4 次退避重试），
  仍失败则停下并标记 partial，换线路后手动点开始即可续传。
- 画廊目录清单 .ehdl.json：记录 gid/token/总页数/已完成页，
  即使 state.json 丢失也能算出「应该多少张、还差多少张」。

顺带修复：
- Start() 双重加锁导致死锁（引擎完全无法启动）
- clampInt 越界时返回默认值而非边界值（设 30 会静默变 8）
- 直链正则写死 hath.network，CDN 换域名会全军覆没 -> 加图片 URL 兜底
- ReconcileTask 会误报「有改动」

测试从 37 增至 62，其中并发/限流/扫尾用 httptest 模拟 EH 实测：
- 页并发峰值 = 8、画廊并发峰值 = 5（完全生效）
- 12 次请求限流 80ms 实测 966ms（期望 880ms）
- 断点续传 4 张已存在时只发 5 次请求

frontend/js/api.js

@@ -30,12 +30,16 @@ window.eh = {
 
   // ---- 引擎 ----
   status: async () => requireGo().Status(),
+  engineInfo: async () => requireGo().EngineInfo(),
   start: async (payload) => {
     const gids = (payload && payload.onlyGids) || [];
     return requireGo().Start(gids);
   },
   stop: async () => requireGo().Stop(),
 
+  // ---- 扫描下载目录（用 .ehdl.json 清单补全丢失的任务信息）----
+  scanDownloadDir: async () => requireGo().ScanDownloadDir(),
+
   // ---- 系统 ----
   pickDir: async () => requireGo().PickDir(),
   exportFailed: async (gid) => {

frontend/js/renderer.js

@@ -134,17 +134,28 @@ async function loadSettings() {
   $('set-dir').value = settings.downloadRoot || '';
   $('set-proxy').value = settings.proxy || '';
   $('set-proxymode').value = settings.proxyMode || 'off';
-  $('set-conc').value = settings.concurrency || 3;
-  $('set-conc-v').textContent = settings.concurrency || 3;
+  $('set-gconc').value = settings.galleryConcurrency || 5;
+  $('set-gconc-v').textContent = settings.galleryConcurrency || 5;
+  $('set-pconc').value = settings.pageConcurrency || 8;
+  $('set-pconc-v').textContent = settings.pageConcurrency || 8;
+  $('set-interval').value = Math.round((settings.requestIntervalMs || 0) / 1000);
+  $('set-interval-v').textContent = Math.round((settings.requestIntervalMs || 0) / 1000);
   $('set-to').value = Math.round((settings.timeout || 30000) / 1000);
   $('set-to-v').textContent = Math.round((settings.timeout || 30000) / 1000);
   $('set-xres').value = String(settings.xres || 'auto');
   $('set-bridgeport').value = String(settings.bridgePort || 37777);
   $('set-dirtpl').value = settings.dirTemplate || '{title}';
+  updateMaxConcurrent();
   renderPresets();
   refreshPreview();
 }
 
+function updateMaxConcurrent() {
+  const g = parseInt($('set-gconc').value, 10) || 5;
+  const p = parseInt($('set-pconc').value, 10) || 8;
+  $('set-max').textContent = g * p;
+}
+
 const DIR_PRESETS = [
   ['{title}', '仅标题'],
   ['{artist}/{title}', '按作者'],
@@ -208,7 +219,15 @@ function bind() {
     if (d) $('set-dir').value = d;
   };
 
-  $('set-conc').oninput = (e) => { $('set-conc-v').textContent = e.target.value; };
+  $('set-gconc').oninput = (e) => {
+    $('set-gconc-v').textContent = e.target.value;
+    updateMaxConcurrent();
+  };
+  $('set-pconc').oninput = (e) => {
+    $('set-pconc-v').textContent = e.target.value;
+    updateMaxConcurrent();
+  };
+  $('set-interval').oninput = (e) => { $('set-interval-v').textContent = e.target.value; };
   $('set-to').oninput = (e) => { $('set-to-v').textContent = e.target.value; };
   $('set-dirtpl').oninput = () => { renderPresets(); refreshPreview(); };
 
@@ -293,7 +312,9 @@ async function saveSettings() {
     proxy: $('set-proxy').value.trim(),
     proxyMode: $('set-proxymode').value,
     xres: $('set-xres').value,
-    concurrency: parseInt($('set-conc').value, 10),
+    galleryConcurrency: parseInt($('set-gconc').value, 10),
+    pageConcurrency: parseInt($('set-pconc').value, 10),
+    requestIntervalMs: parseInt($('set-interval').value, 10) * 1000,
     timeout: parseInt($('set-to').value, 10) * 1000,
     bridgePort: parseInt($('set-bridgeport').value, 10) || 37777,
     dirTemplate: $('set-dirtpl').value.trim() || '{title}',
@@ -301,7 +322,13 @@ async function saveSettings() {
   await window.eh.setSettings(patch);
   $('set-msg').textContent = '';
   closeModal('modal-settings');
-  pushLog({ level: 'ok', msg: '设置已保存', at: Date.now() });
+  const max = patch.galleryConcurrency * patch.pageConcurrency;
+  pushLog({
+    level: 'ok',
+    msg: `设置已保存 · 画廊并发 ${patch.galleryConcurrency} × 页并发 ${patch.pageConcurrency}（最多 ${max} 张同时下载）` +
+         (patch.requestIntervalMs ? ` · 请求间隔 ${patch.requestIntervalMs / 1000} 秒` : ''),
+    at: Date.now(),
+  });
 }
 
 async function doStart() {