feat: Add invite code field to registration form

Frontend Changes:
- Added inviteCode field to registration schema (optional)
- Added invite code TextField to registration form UI
- Updated RegisterData interface in AuthContext to include inviteCode
- Pass inviteCode to backend during registration
- Added helpful placeholder text indicating field is optional

User Experience:
- Invite code field appears after email in registration form
- Helper text explains it's optional and can be left blank if registration is open
- Backend will validate the code if REGISTRATION_MODE=invite_only

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

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

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

@@ -33,6 +33,7 @@ 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(),
   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, {
@@ -129,6 +130,7 @@ export default function RegisterPage() {
         name: data.name,
         email: data.email,
         password: data.password,
+        inviteCode: data.inviteCode || undefined,
         dateOfBirth: data.dateOfBirth,
         parentalEmail: data.parentalEmail || undefined,
         coppaConsentGiven: data.coppaConsent || false,
@@ -242,6 +244,27 @@ export default function RegisterPage() {
               }}
             />
 
+            <TextField
+              fullWidth
+              label="Invite Code (Optional)"
+              margin="normal"
+              error={!!errors.inviteCode}
+              helperText={errors.inviteCode?.message || 'Leave blank if registration is open'}
+              {...register('inviteCode')}
+              disabled={isLoading}
+              inputProps={{
+                'aria-invalid': !!errors.inviteCode,
+                'aria-describedby': errors.inviteCode ? 'invite-code-error' : 'invite-code-helper',
+              }}
+              InputProps={{
+                sx: { borderRadius: 3 },
+              }}
+              FormHelperTextProps={{
+                id: errors.inviteCode ? 'invite-code-error' : 'invite-code-helper',
+                role: errors.inviteCode ? 'alert' : undefined,
+              }}
+            />
+
             <TextField
               fullWidth
               label="Password"

maternal-web/lib/auth/AuthContext.tsx

@@ -31,6 +31,7 @@ export interface RegisterData {
   password: string;
   name: string;
   role?: string;
+  inviteCode?: string; // Required if REGISTRATION_MODE=invite_only
   dateOfBirth: string; // COPPA compliance - required
   parentalEmail?: string; // For users 13-17
   coppaConsentGiven?: boolean; // For users 13-17
@@ -240,6 +241,11 @@ export const AuthProvider = ({ children }: { children: ReactNode }) => {
         deviceInfo,
       };
 
+      // Add optional invite code if provided
+      if (data.inviteCode) {
+        payload.inviteCode = data.inviteCode;
+      }
+
       // Add optional COPPA fields if provided
       if (data.parentalEmail) {
         payload.parentalEmail = data.parentalEmail;