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 <noreply@anthropic.com>

backend/src/services/emailProcessor.js

@@ -1,4 +1,5 @@
 const nodemailer = require('nodemailer');
+const Handlebars = require('handlebars');
 const { db } = require('../database/db');
 const logger = require('../utils/logger');
 
@@ -138,27 +139,15 @@ async function processTemplate(template, variables, language = 'en') {
   const frontendUrl = process.env.FRONTEND_URL || 'http://localhost:3005';
   const logoFullUrl = logoUrl ? `${apiUrl}${logoUrl}` : `${frontendUrl}/picpeak-logo-transparent.png`;
 
-  // Process welcome message section if present
-  let welcomeMessageSection = '';
-  if (variables.welcome_message && variables.welcome_message.trim() !== '') {
-    const welcomeTitle = language === 'de' ? 'Persönliche Nachricht:' : 'Personal Message:';
-    welcomeMessageSection = `
-<div style="background-color: #f3f4f6; padding: 20px; border-radius: 8px; margin: 20px 0;">
-  <p style="margin: 0 0 10px 0; font-weight: 600; color: #374151;">${welcomeTitle}</p>
-  <p style="margin: 0; color: #4b5563;">${variables.welcome_message}</p>
-</div>`;
-  }
-
-  // Replace variables
-  Object.entries(variables).forEach(([key, value]) => {
-    const regex = new RegExp(`{{${key}}}`, 'g');
-    subject = subject.replace(regex, value || '');
-    htmlBody = htmlBody.replace(regex, value || '');
-    textBody = textBody.replace(regex, value || '');
-  });
-
-  // Replace welcome message section placeholder
-  htmlBody = htmlBody.replace(/{{welcome_message_section}}/g, welcomeMessageSection);
+  // Compile templates with Handlebars
+  const subjectTemplate = Handlebars.compile(subject);
+  const htmlTemplate = Handlebars.compile(htmlBody);
+  const textTemplate = Handlebars.compile(textBody);
+  
+  // Process templates with variables
+  subject = subjectTemplate(variables);
+  htmlBody = htmlTemplate(variables);
+  textBody = textTemplate(variables);
 
   // Wrap HTML body in styled template
   const styledHtmlBody = `