[CORE-178] Userlogin Api Integration

[1457dys]

Type of changes

Feature

Purpose

• Add Google and NYCU OAuth login redirects
• Handle OAuth callback and user fetch after login

Additional Information

• Frontend now assumes allow_onboarding as the allow-list flag for welcome access.
• Backend contract is not fully finalized yet, so this PR includes temporary fallback behavior.
• After backend confirms and always returns the final field, we should remove fallback and enforce the finalized contract strictly.

[elvisdragonmao]

好怪但好喔