feat(download): 添加快手下载器并优化下载配置功能

- 新增快手下载器，支持快手视频下载
- 添加下载配置页面，可设置各平台Cookies
- 优化后端接口，增加获取和更新Cookies的功能
- 前端新增Downloader组件和相关表单组件
- 更新路由配置，增加下载配置相关路由

backend/app/routers/config.py

@@ -0,0 +1,30 @@
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+from typing import Optional
+from app.utils.response import ResponseWrapper as R
+
+from app.services.cookie_manager import CookieConfigManager
+
+router = APIRouter()
+cookie_manager = CookieConfigManager()
+
+
+class CookieUpdateRequest(BaseModel):
+    platform: str
+    cookie: str
+
+
+@router.get("/get_downloader_cookie/{platform}")
+def get_cookie(platform: str):
+    cookie = cookie_manager.get(platform)
+    if not cookie:
+        return R.success(msg='未找到Cookies')
+    return R.success(
+        data={"platform": platform, "cookie": cookie}
+    )
+
+
+@router.post("/update_downloader_cookie")
+def update_cookie(data: CookieUpdateRequest):
+    cookie_manager.set(data.platform, data.cookie)
+    return {"message": "Cookie updated successfully"}