♻️ refactor(auth, ui): simplify Loading component & optimize OAuth2Callback flow

* Removed `prompt` prop from `Loading` and switched to built-in Spin indicator with default size `small`
* Dropped overlay background to make the spinner more reusable
* Replaced custom text span; callers can now supply tip via their own UI if needed
* Cleaned up `OAuth2Callback`:
  - Eliminated unused state/variables
  - Added MAX_RETRIES with incremental back-off
  - Centralized error handling via try/catch
  - Streamlined navigation logic on success/failure
  - Updated imports to match new Loading signature

BREAKING CHANGE: `Loading` no longer accepts a `prompt` prop. Update all invocations accordingly.

web/src/i18n/locales/en.json

@@ -179,7 +179,6 @@
   "注销": "Logout",
   "登录": "Sign in",
   "注册": "Sign up",
-  "加载{name}中...": "Loading {name}...",
   "未登录或登录已过期，请重新登录！": "Not logged in or session expired. Please login again!",
   "用户登录": "User Login",
   "密码": "Password",
@@ -933,7 +932,6 @@
   "更新令牌后需等待几分钟生效": "It will take a few minutes to take effect after updating the token.",
   "一小时": "One hour",
   "新建数量": "New quantity",
-  "加载失败，请稍后重试": "Loading failed, please try again later",
   "未设置": "Not set",
   "API文档": "API documentation",
   "不是合法的 JSON 字符串": "Not a valid JSON string",