fix: modal onClose callback, calendar popup truncation, datetime validation

- modal.js: add onClose callback to openModal(), fix _initialFormTimeout
  cleanup, deduplicate escape/overlay-click handlers in confirmModal,
  promptModal, selectModal using the new callback
- calendar.js: replace popup.innerHTML with insertAdjacentHTML (constraint),
  add truncateDescription() to cap long event descriptions at 500 chars
- validate.js: extend DATETIME_RE to cover ISO 8601 with ms/timezone,
  normalise datetime values to YYYY-MM-DDTHH:MM on input
- index.js: include app version in startup log message
- docker-compose.yml / .env.example: switch from named volume to
  host-mounted DATA_DIR/BACKUP_DIR with sane defaults
- docs/installation.md: document host-mount data paths

Co-Authored-By: Rafael Foster <rafaelfoster@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/index.js

@@ -268,7 +268,7 @@ async function runSync() {
 // Server starten
 // --------------------------------------------------------
 app.listen(PORT, () => {
-  logOikos.info(`Server running on port ${PORT}`);
+  logOikos.info(`Server running on port ${PORT} | Version ${APP_VERSION}`);
   logOikos.info(`Environment: ${process.env.NODE_ENV || 'development'}`);
 
   // Erster Sync nach 10 Sekunden (warten bis DB vollständig initialisiert)