dd Statistics tab with real-time native host monitoring

- Add getProcessStats message handler to native host returning process
  memory (rss, heap used/total, external), PID, uptime, OS memory,
  CPU load averages, core count, platform/arch, and tunnel state counts
- Add stats-tracker.js to background SW tracking cumulative connections,
  reconnect count, last disconnect time, and per-minute connection rate
  ring buffer (30 buckets) for sparkline display
- Include statsHistory and activeConnections array in all getState responses
- Add Statistics dashboard page with six sections: Native Host Process,
  System Resources, Tunnel Health, Proxy Traffic, Peer Connections
  (with SVG sparkline), and Extension & Proxy Info
- Preserve checkbox selection across table re-renders on all bulk-action
  tables (virtual hosts, servers, service tunnels)
- Add theme-aware custom checkbox styling for bulk selection rows using
  design system tokens, with white checkmark override for light theme

extension/dashboard/events.js

@@ -242,4 +242,5 @@ function setupEvents() {
   setupServerEvents();
   setupServiceTunnelEvents();
   setupLogsEvents();
+  setupStatsEvents();
 }