fix: prevent double date formatting in email templates

- Remove pre-formatting of dates before passing to email processor
- Let email processor handle all date formatting based on recipient language
- Fix Invalid Date errors by passing raw date values instead of formatted strings
- Remove unused formatDate imports and variables

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

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

backend/src/routes/events.js

@@ -87,10 +87,10 @@ router.post('/', adminAuth, [
     await queueEmail(eventId, host_email, 'gallery_created', {
       host_name: host_email.split('@')[0], // Extract name from email
       event_name,
-      event_date: new Date(event_date).toLocaleDateString(),
+      event_date: event_date,  // Pass raw date - will be formatted by email processor
       gallery_link: shareLink,
       gallery_password: password,
-      expiry_date: expires_at.toLocaleDateString(),
+      expiry_date: expires_at.toISOString(),  // Pass ISO string - will be formatted by email processor
       welcome_message: welcome_message || ''
     });