fix(admin-events): polish form papercuts

- Hide the location field's static help text when a validation error is
  shown so the two near-identical messages stop stacking.
- Replace `process.client` with `import.meta.client` (Nuxt 3+ pattern).
- Accept either String or Date for EventTicketPurchase.eventStartDate;
  the parent passes the API's ISO string, which was logging a Vue prop
  type warning on every public event page render.

app/components/EventTicketPurchase.vue

@@ -224,7 +224,7 @@ const props = defineProps({
     required: true,
   },
   eventStartDate: {
-    type: Date,
+    type: [String, Date],
     required: true,
   },
   eventTitle: {