♻️ 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/components/common/Loading.js

@@ -1,22 +1,14 @@
 import React from 'react';
 import { Spin } from '@douyinfe/semi-ui';
-import { useTranslation } from 'react-i18next';
 
-const Loading = ({ prompt: name = '', size = 'large' }) => {
-  const { t } = useTranslation();
+const Loading = ({ size = 'small' }) => {
 
   return (
-    <div className="fixed inset-0 w-screen h-screen flex items-center justify-center bg-white/80 z-[1000]">
-      <div className="flex flex-col items-center">
-        <Spin
-          size={size}
-          spinning={true}
-          tip={null}
-        />
-        <span className="whitespace-nowrap mt-2 text-center" style={{ color: 'var(--semi-color-primary)' }}>
-          {name ? t('{{name}}', { name }) : t('加载中...')}
-        </span>
-      </div>
+    <div className="fixed inset-0 w-screen h-screen flex items-center justify-center">
+      <Spin
+        size={size}
+        spinning={true}
+      />
     </div>
   );
 };