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/app.js

@@ -39,6 +39,9 @@ import {
     dashboardToggleProfile, dashboardStartTarget, dashboardStopTarget, dashboardStopAll,
     toggleDashboardSection,
 } from './features/dashboard.js';
+import {
+    startPerfPolling, stopPerfPolling,
+} from './features/perf-charts.js';
 import {
     loadPictureSources, switchStreamTab,
     showAddTemplateModal, editTemplate, closeTemplateModal, saveTemplate, deleteTemplate,
@@ -153,6 +156,8 @@ Object.assign(window, {
     dashboardStopTarget,
     dashboardStopAll,
     toggleDashboardSection,
+    startPerfPolling,
+    stopPerfPolling,
 
     // streams / capture templates / PP templates
     loadPictureSources,