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>

.env.example

@@ -6,6 +6,10 @@ PORT=3000
 NODE_ENV=production
 # LOG_LEVEL=info   # debug, info, warn, error (default: info)
 
+# Docker storage paths
+# DATA_DIR=./data              # Host folder for the database and stored app data
+# BACKUP_DIR=./backups         # Host folder for scheduled backups
+
 # Session
 SESSION_SECRET=REPLACE_WITH_A_LONG_RANDOM_STRING
 # SESSION_SECURE=false   # Only set when not using HTTPS/reverse proxy (e.g. direct localhost)
@@ -36,7 +40,6 @@ SYNC_INTERVAL_MINUTES=15
 # Automatic Backups
 # BACKUP_ENABLED=true                  # Enable/disable automated backups (default: true)
 # BACKUP_SCHEDULE=0 2 * * *            # Cron schedule (default: 2 AM daily)
-# BACKUP_DIR=./backups                 # Backup directory (default: ./backups)
 # BACKUP_KEEP=7                        # Number of backups to keep (default: 7)
 # TZ=Europe/Berlin                     # Timezone for scheduled backups (default: UTC)