feat: add api_input LED interpolation; fix LED preview, FPS charts, dashboard layout

API Input:
- Add interpolation mode (linear/nearest/none) for LED count mismatch
  between incoming data and device LED count
- New IconSelect in editor, i18n for en/ru/zh
- Mark crossfade as won't-do (client owns temporal transitions)
- Mark last-write-wins as already implemented

LED Preview:
- Fix zone-mode preview parsing composite wire format (0xFE header
  bytes were rendered as color data, garbling multi-zone previews)
- Fix _restoreLedPreviewState to handle zone-mode panels

FPS Charts:
- Seed target card charts from server metrics-history on first load
- Add fetchMetricsHistory() with 5s TTL cache shared across
  dashboard, targets, perf-charts, and graph-editor
- Fix chart padding: pass maxSamples per caller (120 for dashboard,
  30 for target cards) instead of hardcoded 120
- Fix dashboard chart empty on tab switch (always fetch server history)
- Left-pad with nulls for consistent chart width across targets

Dashboard:
- Fix metrics row alignment (grid layout with fixed column widths)
- Fix FPS label overflow into uptime column

server/src/wled_controller/static/js/features/dashboard.ts

@@ -3,7 +3,7 @@
  */
 
 import { apiKey, _dashboardLoading, set_dashboardLoading, dashboardPollInterval, setDashboardPollInterval, colorStripSourcesCache, devicesCache, outputTargetsCache } from '../core/state.ts';
-import { API_BASE, getHeaders, fetchWithAuth, escapeHtml } from '../core/api.ts';
+import { API_BASE, getHeaders, fetchWithAuth, escapeHtml, fetchMetricsHistory } from '../core/api.ts';
 import { t } from '../core/i18n.ts';
 import { showToast, showConfirm, formatUptime, formatCompact, setTabRefreshing } from '../core/ui.ts';
 import { renderPerfSection, initPerfCharts, startPerfPolling, stopPerfPolling } from './perf-charts.ts';
@@ -99,21 +99,16 @@ function _createFpsChart(canvasId: string, actualHistory: number[], currentHisto
 async function _initFpsCharts(runningTargetIds: string[]): Promise<void> {
     _destroyFpsCharts();
 
-    // Seed FPS history from server ring buffer on first load
-    if (Object.keys(_fpsHistory).length === 0 && runningTargetIds.length > 0) {
-        try {
-            const resp = await fetch(`${API_BASE}/system/metrics-history`, { headers: getHeaders() });
-            if (resp.ok) {
-                const data = await resp.json();
-                const serverTargets = data.targets || {};
-                for (const id of runningTargetIds) {
-                    const samples = serverTargets[id] || [];
-                    _fpsHistory[id] = samples.map(s => s.fps).filter(v => v != null);
-                    _fpsCurrentHistory[id] = samples.map(s => s.fps_current).filter(v => v != null);
-                }
+    // Seed FPS history from server ring buffer (on first load and tab switches)
+    if (runningTargetIds.length > 0) {
+        const data = await fetchMetricsHistory();
+        if (data) {
+            const serverTargets = data.targets || {};
+            for (const id of runningTargetIds) {
+                const samples = serverTargets[id] || [];
+                _fpsHistory[id] = samples.map(s => s.fps).filter(v => v != null);
+                _fpsCurrentHistory[id] = samples.map(s => s.fps_current).filter(v => v != null);
             }
-        } catch {
-            // Silently ignore — charts will fill from polling
         }
     }
 
@@ -162,15 +157,18 @@ function _updateRunningMetrics(enrichedRunning: any[]): void {
         if (chart) {
             const actualH = _fpsHistory[target.id] || [];
             const currentH = _fpsCurrentHistory[target.id] || [];
-            // Mutate in-place to avoid array copies
+            // Left-pad with nulls so all charts span full width
+            const pad0 = MAX_FPS_SAMPLES - actualH.length;
+            const pad1 = MAX_FPS_SAMPLES - currentH.length;
             const ds0 = chart.data.datasets[0].data;
             ds0.length = 0;
+            if (pad0 > 0) for (let i = 0; i < pad0; i++) ds0.push(null);
             ds0.push(...actualH);
             const ds1 = chart.data.datasets[1].data;
             ds1.length = 0;
+            if (pad1 > 0) for (let i = 0; i < pad1; i++) ds1.push(null);
             ds1.push(...currentH);
-            while (chart.data.labels.length < ds0.length) chart.data.labels.push('');
-            chart.data.labels.length = ds0.length;
+            chart.data.labels.length = MAX_FPS_SAMPLES;
             chart.update('none');
         }