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/browser/login.py

@@ -56,9 +56,12 @@ def main():
         page.goto("https://3d.hunyuan.tencent.com/", timeout=60000)
         page.wait_for_timeout(2000)
 
-        # 检查是否已经是登录状态（没有登录按钮说明已登录）
+        # 检查是否已经是登录状态：URL 不含 login 且页面上没有登录按钮
+        is_login_page = "login" in page.url
         login_btn = page.locator("button").filter(has_text="登录").first
-        if login_btn.count() == 0:
+        has_login_btn = login_btn.count() > 0
+
+        if not is_login_page and not has_login_btn:
             print("检测到已有登录状态，无需重新登录。")
             print(f"当前页面: {page.url}")
             _extract_and_save_cookies(context)
@@ -70,6 +73,10 @@ def main():
             login_btn.click()
             page.locator("text=邮箱").wait_for(state="visible", timeout=10000)
 
+            # 定位元素
+            email_input = page.locator('input[placeholder="请输入邮箱地址"]')
+            code_input = page.locator('input[type="number"][placeholder="请输入邮箱验证码"]')
+
             # 切换到邮箱登录
             email_tab = page.locator("text=邮箱").first
             if email_tab.count() > 0:
@@ -78,10 +85,6 @@ def main():
             else:
                 print("未找到邮箱登录选项")
                 return
-
-            # 定位元素
-            email_input = page.locator('input[placeholder="请输入邮箱地址"]')
-            code_input = page.locator('input[type="number"][placeholder="请输入邮箱验证码"]')
             send_code_btn = page.locator("a.hyc-email-login__send-code")
             login_submit_btn = page.locator("button.hyc-email-login__btn")
             checkbox = page.locator(".t-checkbox__former")