feat: add getBaseUrl function for consistent base URL generation across routes

feat(calendar): display local timezone in calendar view
feat(i18n): add timezone label to German and English translations

server/routes/admin.js

@@ -2,7 +2,7 @@
 import bcrypt from 'bcryptjs';
 import { v4 as uuidv4 } from 'uuid';
 import { getDb } from '../config/database.js';
-import { authenticateToken, requireAdmin } from '../middleware/auth.js';
+import { authenticateToken, requireAdmin, getBaseUrl } from '../middleware/auth.js';
 import { isMailerConfigured, sendInviteEmail } from '../config/mailer.js';
 import { log } from '../config/logger.js';
 import {
@@ -208,7 +208,7 @@ router.post('/invites', authenticateToken, requireAdmin, async (req, res) => {
     );
 
     // Send invite email if SMTP is configured
-    const baseUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
+    const baseUrl = getBaseUrl(req);
     const inviteUrl = `${baseUrl}/register?invite=${token}`;
 
     // Load app name