feat: HA-Termine erstellen über calendar.create_event Service

- POST /api/homeassistant/events Endpoint mit calendar.create_event
- Frontend: HA-Termine erstellen statt 'nicht unterstützt' Toast
- Datetime-Format an HA-Konvention angepasst:
  'YYYY-MM-DD HH:MM:SS' (Space-Separator, ohne Timezone)
- _ha_format_dt Helper für ISO → HA Datetime-Konvertierung

frontend/js/calendar.js

@@ -1460,8 +1460,12 @@ function bindEventModal() {
         });
         showToast(t('event_created'));
       } else if (isHA) {
-        showToast(t('ha_create_not_supported'), true);
-        return;
+        const haCalId = parseInt(calVal.replace('homeassistant-', ''));
+        await api.post('/homeassistant/events', {
+          calendar_id: haCalId, title, start, end, allDay,
+          location: loc, description: desc,
+        });
+        showToast(t('event_created'));
       } else {
         const calId = parseInt(calVal);
         await api.post('/caldav/events', {