Implement logged-in users monitoring and improve widget formatting

Agent improvements:
- Add get_logged_in_users() function to SystemCollector using 'who' command
- Collect unique, sorted list of currently logged-in users
- Include logged_in_users field in system metrics JSON output
- Change C-state formatting to show 2 states per row instead of 4

Dashboard improvements:
- Update Backups widget to show "Archives: XX, ..." format
- System widget ready to display logged-in users with proper formatting

The System widget will now show:
- C-states formatted as 2 per row for better readability
- Logged-in users displayed as "Logged in: user" or "Logged in: X users (user1, user2)"

dashboard/src/ui/backup.rs

@@ -59,7 +59,7 @@ fn render_metrics(frame: &mut Frame, _host: &HostDisplayData, metrics: &BackupMe
 
     data.add_row(
         Some(WidgetStatus::new(latest_status)),
-        vec![format!("{} archives, {:.1}GB total", metrics.backup.snapshot_count, metrics.backup.size_gb)],
+        vec![format!("Archives: {}, {:.1}GB total", metrics.backup.snapshot_count, metrics.backup.size_gb)],
         vec![
             "Latest".to_string(),
             latest_time,