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/middleware/auth.js

@@ -57,3 +57,14 @@ export function generateToken(userId) {
   const jti = uuidv4();
   return jwt.sign({ userId, jti }, JWT_SECRET, { expiresIn: '7d' });
 }
+
+/**
+ * Build the public base URL for the application.
+ * Prefers APP_URL env var. Falls back to X-Forwarded-Proto + Host header
+ * so that links are correct behind a TLS-terminating reverse proxy.
+ */
+export function getBaseUrl(req) {
+  if (process.env.APP_URL) return process.env.APP_URL.replace(/\/+$/, '');
+  const proto = req.get('x-forwarded-proto')?.split(',')[0]?.trim() || req.protocol;
+  return `${proto}://${req.get('host')}`;
+}