Feat: Add /login/channels route and improve auth logic for frontend third-party login integration (#7521)

### What problem does this PR solve?

Add `/login/channels` route and improve auth logic to support frontend
integration with third-party login providers:

- Add `/login/channels` route to provide authentication channel list
with `display_name` and `icon`
- Optimize user info parsing logic by prioritizing `avatar_url` and
falling back to `picture`
- Simplify OIDC token validation by removing unnecessary `kid` checks
- Ensure `client_id` is safely cast to string during `audience`
validation
- Fix typo

---
- Related pull request: #7379 

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

api/apps/auth/oauth.py

@@ -102,5 +102,7 @@ class OAuthClient:
         email = user_info.get("email")
         username = user_info.get("username", str(email).split("@")[0])
         nickname = user_info.get("nickname", username)
-        avatar_url = user_info.get("picture", "")
+        avatar_url = user_info.get("avatar_url", None)
+        if avatar_url is None:
+            avatar_url = user_info.get("picture", "")
         return UserInfo(email=email, username=username, nickname=nickname, avatar_url=avatar_url)