1.4.2: feat: Add backup status UI to admin settings v1.4.1

- Add Backup Status section to admin settings page
- Display cron schedule, next poll time, and backup progress
- Show system requirements check with detailed feedback
- Add manual backup poll trigger button
- Add view/clear backup logs functionality
- Implement AJAX handlers for backup operations
- Auto-refresh logs after manual trigger
- Add collapsible log viewer with 100 recent entries
- Visual status indicators (✓/✗/⏳)

UI Features:
- Real-time backup status monitoring
- One-click manual backup poll trigger
- Inline log viewer with monospace formatting
- Clear feedback messages for all operations
- Requirements validation display

AJAX Endpoints:
- errorvault_trigger_backup_poll
- errorvault_get_backup_logs
- errorvault_clear_backup_logs