Add real-time system performance charts to dashboard

Backend: GET /api/v1/system/performance endpoint using psutil (CPU/RAM)
and nvidia-ml-py (GPU utilization, memory, temperature) with graceful
fallback. Frontend: Chart.js line charts with rolling 60-sample history
persisted to sessionStorage, flicker-free updates via persistent DOM
and diff-based dynamic section refresh.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/features/tabs.js

@@ -14,6 +14,7 @@ export function switchTab(name) {
         if (typeof window.startDashboardWS === 'function') window.startDashboardWS();
     } else {
         if (typeof window.stopDashboardWS === 'function') window.stopDashboardWS();
+        if (typeof window.stopPerfPolling === 'function') window.stopPerfPolling();
         if (name === 'streams') {
             if (typeof window.loadPictureSources === 'function') window.loadPictureSources();
         } else if (name === 'targets') {