fix(sign-in): validate email format before registration attempt

Show "Please enter a valid email address" when the user enters a
username instead of an email in the sign-up form, rather than letting
it hit Logto's API and returning a cryptic 400.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/sign-in/src/SignInPage.tsx

@@ -103,6 +103,10 @@ export function SignInPage() {
   const handleRegister = async (e: FormEvent) => {
     e.preventDefault();
     setError(null);
+    if (!identifier.includes('@')) {
+      setError('Please enter a valid email address');
+      return;
+    }
     if (password !== confirmPassword) {
       setError('Passwords do not match');
       return;