Add a new filter button that shows only guests with stale, critical,
or missing backups. This makes it easy to identify which VMs and
containers need attention for backup scheduling.
- Adds backupMode state with 'all' and 'needs-backup' options
- Filters out templates (they don't need backups)
- Uses existing getBackupInfo() thresholds (>24h stale, >72h critical)
- Integrates with Reset button and Escape key handling
- Persists filter state in localStorage
Related to #762
b70a4eba9d