fix(kiosk): Filter empty optional fields before API submission

- Prevents validation errors when email/phone/company are empty strings
- Only sends fields with actual values to backend

frontend/src/pages/VisitorKioskPage.tsx

@@ -63,7 +63,17 @@ export default function VisitorKioskPage() {
         setError(null);
 
         try {
-            const visitor = await visitorsApi.create(formData);
+            // Only include non-empty fields to avoid validation errors on optional fields
+            const visitorData: Record<string, any> = {
+                name: formData.name,
+                purpose: formData.purpose,
+                type: formData.type
+            };
+            if (formData.company) visitorData.company = formData.company;
+            if (formData.email) visitorData.email = formData.email;
+            if (formData.phone) visitorData.phone = formData.phone;
+
+            const visitor = await visitorsApi.create(visitorData);
             const result = await visitorsApi.checkIn(visitor.id, {
                 ndaAccepted: formData.ndaAccepted
             });