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/style.css

@@ -3416,6 +3416,57 @@ input:-webkit-autofill:focus {
     }
 }
 
+/* ===== PERFORMANCE CHARTS ===== */
+
+.perf-charts-grid {
+    display: grid;
+    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
+    gap: 12px;
+}
+
+.perf-chart-card {
+    background: var(--card-bg);
+    border: 1px solid var(--border-color);
+    border-radius: 6px;
+    padding: 10px 12px;
+}
+
+.perf-chart-wrap {
+    position: relative;
+    height: 60px;
+}
+
+.perf-chart-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    margin-bottom: 4px;
+}
+
+.perf-chart-label {
+    font-size: 0.75rem;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.3px;
+    color: var(--text-secondary);
+}
+
+.perf-chart-value {
+    font-size: 0.85rem;
+    font-weight: 700;
+}
+
+.perf-chart-value.cpu { color: #2196F3; }
+.perf-chart-value.ram { color: #4CAF50; }
+.perf-chart-value.gpu { color: #FF9800; }
+
+.perf-chart-unavailable {
+    text-align: center;
+    padding: 20px 0;
+    color: var(--text-secondary);
+    font-size: 0.8rem;
+}
+
 /* ===== PROFILES ===== */
 
 .badge-profile-active {