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

src/i18n/de.json

@@ -507,6 +507,7 @@
     "linkedRoom": "Verknüpfter Raum",
     "noRoom": "Kein Raum (kein Videomeeting)",
     "linkedRoomHint": "Verknüpfe einen Raum, um die Beitritts-URL automatisch ins Event einzufügen.",
+    "timezone": "Zeitzone",
     "color": "Farbe",
     "eventCreated": "Event erstellt!",
     "eventUpdated": "Event aktualisiert!",

src/i18n/en.json

@@ -507,6 +507,7 @@
     "linkedRoom": "Linked Room",
     "noRoom": "No room (no video meeting)",
     "linkedRoomHint": "Link a room to automatically include the join-URL in the event.",
+    "timezone": "Timezone",
     "color": "Color",
     "eventCreated": "Event created!",
     "eventUpdated": "Event updated!",

src/pages/Calendar.jsx

@@ -554,6 +554,9 @@ export default function Calendar() {
                 />
               </div>
             </div>
+            <p className="text-xs text-th-text-s -mt-2">
+              {t('calendar.timezone')}: {getLocalTimezone()}
+            </p>
 
             <div>
               <label className="block text-sm font-medium text-th-text mb-1.5">{t('calendar.linkedRoom')}</label>
@@ -606,6 +609,7 @@ export default function Calendar() {
               <span>
                 {new Date(showDetail.start_time).toLocaleString()} - {new Date(showDetail.end_time).toLocaleString()}
               </span>
+              <span className="text-xs opacity-70">({getLocalTimezone()})</span>
             </div>
 
             {showDetail.description && (
@@ -846,3 +850,15 @@ function formatTime(dateStr) {
   const d = new Date(dateStr);
   return d.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' });
 }
+
+function getLocalTimezone() {
+  try {
+    return Intl.DateTimeFormat().resolvedOptions().timeZone;
+  } catch {
+    const offset = -new Date().getTimezoneOffset();
+    const sign = offset >= 0 ? '+' : '-';
+    const h = String(Math.floor(Math.abs(offset) / 60)).padStart(2, '0');
+    const m = String(Math.abs(offset) % 60).padStart(2, '0');
+    return `UTC${sign}${h}:${m}`;
+  }
+}