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>

hunyuan3dweb/api.py

@@ -34,8 +34,12 @@ class Hunyuan3DAPI:
             "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36"
         })
 
-        if cookies:
-            self.set_cookies(cookies)
+        try:
+            cookie_value = cookies if cookies is not None else load_cookies_from_file()
+        except FileNotFoundError:
+            cookie_value = None
+        if cookie_value:
+            self.set_cookies(cookie_value)
 
     def set_cookies(self, cookies: str):
         """设置Cookie"""