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/wailsjs/go/main/App.d.ts

@@ -11,6 +11,8 @@ export function ClearFinished():Promise<Record<string, any>>;
 
 export function CountDone():Promise<number>;
 
+export function EngineInfo():Promise<main.EngineInfo>;
+
 export function ExportFailed(arg1:string):Promise<string>;
 
 export function GetSettings():Promise<core.Settings>;
@@ -29,6 +31,8 @@ export function RemoveTask(arg1:string,arg2:string):Promise<Array<core.Task>>;
 
 export function RequeueFailed():Promise<Array<core.Task>>;
 
+export function ScanDownloadDir():Promise<main.ScanManifestsResult>;
+
 export function SelectAll(arg1:Array<string>):Promise<Array<core.Task>>;
 
 export function SetSettings(arg1:Record<string, any>):Promise<core.Settings>;

frontend/wailsjs/go/main/App.js

@@ -18,6 +18,10 @@ export function CountDone() {
   return window['go']['main']['App']['CountDone']();
 }
 
+export function EngineInfo() {
+  return window['go']['main']['App']['EngineInfo']();
+}
+
 export function ExportFailed(arg1) {
   return window['go']['main']['App']['ExportFailed'](arg1);
 }
@@ -54,6 +58,10 @@ export function RequeueFailed() {
   return window['go']['main']['App']['RequeueFailed']();
 }
 
+export function ScanDownloadDir() {
+  return window['go']['main']['App']['ScanDownloadDir']();
+}
+
 export function SelectAll(arg1) {
   return window['go']['main']['App']['SelectAll'](arg1);
 }

frontend/wailsjs/go/models.ts

@@ -20,7 +20,9 @@ export namespace core {
 	    downloadRoot: string;
 	    proxy: string;
 	    proxyMode: string;
-	    concurrency: number;
+	    galleryConcurrency: number;
+	    pageConcurrency: number;
+	    requestIntervalMs: number;
 	    timeout: number;
 	    xres: string;
 	    bridgePort: number;
@@ -36,7 +38,9 @@ export namespace core {
 	        this.downloadRoot = source["downloadRoot"];
 	        this.proxy = source["proxy"];
 	        this.proxyMode = source["proxyMode"];
-	        this.concurrency = source["concurrency"];
+	        this.galleryConcurrency = source["galleryConcurrency"];
+	        this.pageConcurrency = source["pageConcurrency"];
+	        this.requestIntervalMs = source["requestIntervalMs"];
 	        this.timeout = source["timeout"];
 	        this.xres = source["xres"];
 	        this.bridgePort = source["bridgePort"];
@@ -139,6 +143,46 @@ export namespace main {
 	        this.error = source["error"];
 	    }
 	}
+	export class EngineInfo {
+	    running: boolean;
+	    queued: number;
+	    galleryConcurrency: number;
+	    pageConcurrency: number;
+	    maxConcurrent: number;
+	    requestIntervalMs: number;
+	
+	    static createFrom(source: any = {}) {
+	        return new EngineInfo(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.running = source["running"];
+	        this.queued = source["queued"];
+	        this.galleryConcurrency = source["galleryConcurrency"];
+	        this.pageConcurrency = source["pageConcurrency"];
+	        this.maxConcurrent = source["maxConcurrent"];
+	        this.requestIntervalMs = source["requestIntervalMs"];
+	    }
+	}
+	export class ScanManifestsResult {
+	    found: number;
+	    adopted: number;
+	    root: string;
+	    note: string;
+	
+	    static createFrom(source: any = {}) {
+	        return new ScanManifestsResult(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.found = source["found"];
+	        this.adopted = source["adopted"];
+	        this.root = source["root"];
+	        this.note = source["note"];
+	    }
+	}
 	export class StartResult {
 	    started: boolean;
 	    reason: string;