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/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!",