feat: add resend creation email button and fix Handlebars template processing

- Fixed email template processing to properly handle Handlebars conditionals
  - Installed handlebars package
  - Updated processTemplate to use Handlebars.compile()
  - Now {{#if welcome_message}} blocks work correctly

- Added "Resend Creation Email" button to event details page
  - New endpoint: POST /api/admin/events/:id/resend-email
  - Button appears below "Reset Gallery Password"
  - Shows mail icon and includes success/error notifications
  - For security, resent emails show "(Aus Sicherheitsgründen nicht angezeigt)"
    instead of the actual password

- Added translations:
  - EN: "Resend Creation Email", success/error messages
  - DE: "Erstellungs-E-Mail erneut senden", success/error messages

This fixes the issue where Handlebars syntax was showing in emails and provides
a convenient way to resend the initial gallery creation email to hosts.

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

Co-Authored-By: Claude <[email protected]>

frontend/src/pages/admin/EventDetailsPage.tsx

@@ -15,7 +15,8 @@ import {
   CheckCircle,
   Upload,
   Image,
-  Key
+  Key,
+  Mail
 } from 'lucide-react';
 import { parseISO, differenceInDays } from 'date-fns';
 import { toast } from 'react-toastify';
@@ -620,7 +621,7 @@ export const EventDetailsPage: React.FC = () => {
             </p>
             
             {!event.is_archived && (
-              <div className="mt-4 pt-4 border-t border-neutral-200">
+              <div className="mt-4 pt-4 border-t border-neutral-200 space-y-2">
                 <Button
                   variant="outline"
                   size="sm"
@@ -630,6 +631,22 @@ export const EventDetailsPage: React.FC = () => {
                 >
                   {t('events.resetGalleryPassword')}
                 </Button>
+                <Button
+                  variant="outline"
+                  size="sm"
+                  leftIcon={<Mail className="w-4 h-4" />}
+                  onClick={async () => {
+                    try {
+                      await eventsService.resendCreationEmail(event.id);
+                      toast.success(t('events.creationEmailResent'));
+                    } catch (error) {
+                      toast.error(t('events.failedToResendEmail'));
+                    }
+                  }}
+                  className="w-full justify-center"
+                >
+                  {t('events.resendCreationEmail')}
+                </Button>
               </div>
             )}
           </Card>