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/cmd/server/main.go

@@ -549,6 +549,7 @@ func main() {
 	// Signup Routes
 	signup := auth.Group("/signup")
 	signup.Post("/check-email", authHandler.CheckEmail)
+	signup.Post("/check-login-id", authHandler.CheckLoginID)
 	signup.Post("/send-email-code", authHandler.SendSignupEmailCode)
 	signup.Post("/send-sms-code", authHandler.SendSignupSmsCode)
 	signup.Post("/verify-code", authHandler.VerifySignupCode)