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/federation.js

@@ -2,7 +2,7 @@
 import { v4 as uuidv4 } from 'uuid';
 import { rateLimit } from 'express-rate-limit';
 import { getDb } from '../config/database.js';
-import { authenticateToken } from '../middleware/auth.js';
+import { authenticateToken, getBaseUrl } from '../middleware/auth.js';
 import { sendFederationInviteEmail, sendCalendarEventDeletedEmail } from '../config/mailer.js';
 import { log } from '../config/logger.js';
 import { createNotification } from '../config/notifications.js';
@@ -84,7 +84,7 @@ router.post('/invite', authenticateToken, async (req, res) => {
 
         // Build guest join URL for the remote user
         // If the room has an access code, embed it so the recipient can join without manual entry
-        const baseUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
+        const baseUrl = getBaseUrl(req);
         const joinUrl = room.access_code
             ? `${baseUrl}/join/${room.uid}?ac=${encodeURIComponent(room.access_code)}`
             : `${baseUrl}/join/${room.uid}`;
@@ -236,7 +236,7 @@ router.post('/receive', federationReceiveLimiter, async (req, res) => {
 
         // Send notification email (truly fire-and-forget - never blocks the response)
         if (targetUser.email) {
-          const appUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
+          const appUrl = getBaseUrl(req);
           const inboxUrl = `${appUrl}/federation/inbox`;
           const appName = process.env.APP_NAME || 'Redlight';
           sendFederationInviteEmail(