- Add `ValidateLoginID` to enforce ID collision and security rules (prevents phone number collision, email format usage, and reserved words).
- Add `POST /api/v1/auth/signup/check-login-id` endpoint for real-time ID availability checks.
- Add `checkLoginIDAvailability` API call to userfront's `AuthProxyService`.
- Implement "Check Duplication" button and error/success messaging for the Login ID field in the signup screen.
- Add "000000" magic code bypass for `VerifySignupCode` in non-production environments to streamline testing.
- Revert the removal of loginId from adminfront and backend.
- Prevent phone normalization logic from mangling custom employee ID login fields.
- Add an explicit 'loginId' optional input field to the userfront signup UI.
- Update AuthProxyService.signup and backend AuthHandler.Signup to transmit and map the 'loginId' parameter properly.
