feat(dashboard): more System History metrics, persistence & localized labels

- Sample swap %, TCP/UDP connection counts and disk-usage % on the host ticker
- System History: Swap overlaid on the RAM tab, plus new Connections and Disk Usage tabs
- Persist the host time-series across restarts: gob snapshot beside the DB, written on a timer and at shutdown, restored on boot
- Live-refresh the open chart (2s for short ranges, 10s for longer)
- Localize CPU/RAM/Swap and the new tab/chart titles across all 13 languages and route legend series names through i18n

web/web.go

@@ -469,6 +469,9 @@ func (s *Server) stop(stopXray bool, stopTgBot bool) error {
 	if s.cron != nil {
 		s.cron.Stop()
 	}
+	if err := service.PersistSystemMetrics(); err != nil {
+		logger.Warning("persist system metrics on shutdown failed:", err)
+	}
 	if stopXray {
 		service.StopTrafficWriter()
 	}