feat: add pure Python COS upload and improve login detection

- feat(cos): add cos_upload.py for direct file upload without browser
  - implements COS V1 signature algorithm with temporary credentials
  - upload_image() pipeline: get_upload_info → sign → PUT to COS
- feat(api): auto-load cookies from file when cookies arg is omitted
  - both Hunyuan3DAPI and Hunyuan3DAPIComplete now fall back to
    ~/.config/hunyuan3dweb/cookies.txt automatically
- fix(login): strengthen login-state detection using both URL and DOM
  - checks "login" not in page.url AND no login button on page
- docs: update README / README_CN with COS upload examples

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README_CN.md

@@ -87,7 +87,7 @@ if result["status"] == "success":
 
 ### 2. 图生3D（本地文件）
 
-本地图片文件需通过浏览器自动化工具上传。纯 API 客户端无法直接上传图片，因为腾讯 COS 临时凭证需要浏览器端解密。
+本地图片文件可通过浏览器自动化工具上传，也可使用纯 Python 的 COS 上传模块先拿到 `resourceUrl` 再走 API。
 
 ```bash
 hunyuan3dweb-generate /path/to/image.png wait
@@ -102,6 +102,18 @@ result = generate_3d("/path/to/image.png", wait_for_complete=True)
 print(f"模型地址: {result.get('modelUrl')}")
 ```
 
+或通过纯 Python 上传：
+
+```python
+from hunyuan3dweb.cos_upload import upload_image
+from hunyuan3dweb import Hunyuan3DAPIComplete
+
+resource_url = upload_image("/path/to/image.png")
+
+api = Hunyuan3DAPIComplete()
+result = api.generate_from_image(resource_url, title="我的模型")
+```
+
 ### 3. 图生3D（API，已有 resourceUrl）
 
 如果你已经有 `resourceUrl`（例如之前通过浏览器上传过），可直接调用 API：
@@ -283,6 +295,7 @@ hunyuan3dweb-sniffer
 | `hunyuan3dweb/api.py` | 基础 API 客户端（图生3D、文生3D） |
 | `hunyuan3dweb/api_complete.py` | 完整 API 客户端（所有生成模式） |
 | `hunyuan3dweb/sign.py` | 腾讯混元3D 签名算法 |
+| `hunyuan3dweb/cos_upload.py` | 纯 Python COS 上传工具 |
 | `hunyuan3dweb/config.py` | 用户配置路径管理 |
 | `hunyuan3dweb/cli.py` | CLI 入口 |
 | `hunyuan3dweb/browser/login.py` | 浏览器登录工具 |