feat: Add client-side validation for required invite code

- Add validation in onSubmit to check for invite code when required
- Display user-friendly error message instead of API error
- Prevent form submission until invite code is provided

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

maternal-web/app/(auth)/register/page.tsx

@@ -25,7 +25,8 @@ import Link from 'next/link';
 import { useTheme } from '@mui/material/styles';
 import apiClient from '@/lib/api/client';
 
-const registerSchema = z.object({
+// Create a function to generate schema dynamically based on requireInviteCode
+const createRegisterSchema = (requireInviteCode: boolean) => z.object({
   name: z.string().min(2, 'Name must be at least 2 characters'),
   email: z.string().email('Invalid email address'),
   password: z.string()
@@ -34,7 +35,9 @@ const registerSchema = z.object({
     .regex(/[a-z]/, 'Password must contain at least one lowercase letter')
     .regex(/[0-9]/, 'Password must contain at least one number'),
   confirmPassword: z.string(),
-  inviteCode: z.string().optional(),
+  inviteCode: requireInviteCode
+    ? z.string().min(1, 'Invite code is required')
+    : z.string().optional(),
   dateOfBirth: z.string().min(1, 'Date of birth is required'),
   parentalEmail: z.string().email('Invalid email address').optional().or(z.literal('')),
   agreeToTerms: z.boolean().refine(val => val === true, {
@@ -70,7 +73,7 @@ const registerSchema = z.object({
   path: ['dateOfBirth'],
 });
 
-type RegisterFormData = z.infer<typeof registerSchema>;
+type RegisterFormData = z.infer<ReturnType<typeof createRegisterSchema>>;
 
 export default function RegisterPage() {
   const theme = useTheme();
@@ -89,8 +92,10 @@ export default function RegisterPage() {
     handleSubmit,
     watch,
     formState: { errors },
+    setValue,
+    trigger,
   } = useForm<RegisterFormData>({
-    resolver: zodResolver(registerSchema),
+    resolver: zodResolver(createRegisterSchema(requireInviteCode)),
     defaultValues: {
       agreeToTerms: false,
       agreeToPrivacy: false,
@@ -146,6 +151,13 @@ export default function RegisterPage() {
 
   const onSubmit = async (data: RegisterFormData) => {
     setError(null);
+
+    // Validate invite code if required
+    if (requireInviteCode && (!data.inviteCode || data.inviteCode.trim() === '')) {
+      setError('Invite code is required to register');
+      return;
+    }
+
     setIsLoading(true);
 
     try {