Add calendar feature: Integrate calendarRouter and CalendarEvent model, enhance CalendarView with API interactions for event management, and update localization files for error handling in both English and German. This update improves the calendar functionality and user experience.

backend/app.js

@@ -22,6 +22,7 @@ import termineRouter from './routers/termineRouter.js';
 import vocabRouter from './routers/vocabRouter.js';
 import dashboardRouter from './routers/dashboardRouter.js';
 import newsRouter from './routers/newsRouter.js';
+import calendarRouter from './routers/calendarRouter.js';
 import cors from 'cors';
 import './jobs/sessionCleanup.js';
 
@@ -82,6 +83,7 @@ app.use('/api/blog', blogRouter);
 app.use('/api/termine', termineRouter);
 app.use('/api/dashboard', dashboardRouter);
 app.use('/api/news', newsRouter);
+app.use('/api/calendar', calendarRouter);
 
 // Serve frontend SPA for non-API routes to support history mode clean URLs
 // /models/* nicht statisch ausliefern – nur über /api/models (Proxy mit Komprimierung)