Add FPS sparkline charts, configurable poll interval, and uptime interpolation

Replace text FPS labels with Chart.js sparklines on running targets,
use emoji icons for metrics, add in-place DOM updates to preserve
chart animations, and add a 1-10s poll interval slider that controls
all dashboard timers. Uptime now ticks every second via client-side
interpolation regardless of poll interval.

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

server/src/wled_controller/static/js/features/perf-charts.js

@@ -4,9 +4,9 @@
 
 import { API_BASE, getHeaders } from '../core/api.js';
 import { t } from '../core/i18n.js';
+import { dashboardPollInterval } from '../core/state.js';
 
 const MAX_SAMPLES = 60;
-const POLL_INTERVAL_MS = 2000;
 const STORAGE_KEY = 'perf_history';
 
 let _pollTimer = null;
@@ -168,7 +168,7 @@ async function _fetchPerformance() {
 export function startPerfPolling() {
     if (_pollTimer) return;
     _fetchPerformance();
-    _pollTimer = setInterval(_fetchPerformance, POLL_INTERVAL_MS);
+    _pollTimer = setInterval(_fetchPerformance, dashboardPollInterval);
 }
 
 export function stopPerfPolling() {