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.md

@@ -87,7 +87,7 @@ if result["status"] == "success":
 
 ### 2. Image-to-3D (Local File)
 
-For local image files, use the browser automation tool. The pure API client cannot upload images directly because Tencent COS requires browser-side decryption of temporary credentials.
+For local image files, you can use either the browser automation tool or the pure Python COS uploader to obtain a `resourceUrl` and then call the 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"Model URL: {result.get('modelUrl')}")
 ```
 
+Or via pure Python upload:
+
+```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="My Model")
+```
+
 ### 3. Image-to-3D (API with Existing resourceUrl)
 
 If you already have a `resourceUrl` (e.g. from a previous browser upload), use the pure API client:
@@ -283,6 +295,7 @@ hunyuan3dweb-sniffer
 | `hunyuan3dweb/api.py` | Basic API client (image2model, text2model) |
 | `hunyuan3dweb/api_complete.py` | Full API client (all generation modes) |
 | `hunyuan3dweb/sign.py` | Tencent Hunyuan 3D signing algorithm |
+| `hunyuan3dweb/cos_upload.py` | Pure Python COS upload helper |
 | `hunyuan3dweb/config.py` | User config path management |
 | `hunyuan3dweb/cli.py` | CLI entry point |
 | `hunyuan3dweb/browser/login.py` | Browser login tool |