feat: add robust login ID collision prevention and UI validation (#440)

- 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.

backend/internal/handler/auth_handler_otp_test.go

@@ -99,7 +99,7 @@ func TestVerifySignupCode_Invalid(t *testing.T) {
 	verifyBody := map[string]string{
 		"type":   "email",
 		"target": "user@test.com",
-		"code":   "000000", // wrong code
+		"code":   "222222", // wrong code
 	}
 	body, _ := json.Marshal(verifyBody)
 	req := httptest.NewRequest(http.MethodPost, "/api/v1/auth/signup/verify", bytes.NewReader(body))