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/pages/admin/events/create.vue

@@ -122,7 +122,7 @@
               <p v-if="fieldErrors.location" class="field-error">
                 {{ fieldErrors.location }}
               </p>
-              <p class="help-text">
+              <p v-if="!fieldErrors.location" class="help-text">
                 Enter a video conference link or Slack channel (starting with #)
               </p>
             </div>
@@ -766,7 +766,7 @@ if (route.query.edit) {
 }
 
 // Check if we're duplicating an event
-if (route.query.duplicate && process.client) {
+if (route.query.duplicate && import.meta.client) {
   const duplicateData = sessionStorage.getItem("duplicateEventData");
   if (duplicateData) {
     try {
@@ -780,7 +780,7 @@ if (route.query.duplicate && process.client) {
 }
 
 // Check if we're creating a series event
-if (route.query.series && process.client) {
+if (route.query.series && import.meta.client) {
   const seriesData = sessionStorage.getItem("seriesEventData");
   if (seriesData) {
     try {