feat: restore explicit loginId field and add to userfront signup flow

- 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.

backend/internal/handler/user_handler.go

@@ -1533,12 +1533,6 @@ func syncLoginID(traits map[string]interface{}, metadata map[string]any, tenantI
 	}
 
 	if loginID != "" {
-		// Normalize if it looks like a phone number to be consistent with other identifiers
-		normalized := normalizePhoneNumber(loginID)
-		if normalized != "" {
-			loginID = normalized
-		}
-
 		slog.Info("Syncing LoginID from custom field", "field", loginIDField, "value", loginID, "tenantID", tenantID)
 		traits["id"] = loginID
 	}