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/locales/en.json

@@ -468,6 +468,11 @@
   "dashboard.failed": "Failed to load dashboard",
   "dashboard.section.profiles": "Profiles",
   "dashboard.targets": "Targets",
+  "dashboard.section.performance": "System Performance",
+  "dashboard.perf.cpu": "CPU",
+  "dashboard.perf.ram": "RAM",
+  "dashboard.perf.gpu": "GPU",
+  "dashboard.perf.unavailable": "unavailable",
 
   "profiles.title": "\uD83D\uDCCB Profiles",
   "profiles.empty": "No profiles configured. Create one to automate target activation.",