[fix](trx-frontend-http): fix TDZ on lastSpectrumData at style init

Same root cause as the overviewDrawPending TDZ: setStyle() references
lastSpectrumData at module init time but it was declared far below.
Hoist let lastSpectrumData = null to the top variable block and remove
the now-duplicate declaration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -333,6 +333,7 @@ const headerStylePickSelect = document.getElementById("header-style-pick-select"
 let overviewPeakHoldMs = Number(loadSetting("overviewPeakHoldMs", 2000));
 
 let overviewDrawPending = false;
+let lastSpectrumData = null;
 let lastControl;
 let lastTxEn = null;
 let lastHasTx = true;
@@ -2723,7 +2724,6 @@ const spectrumFreqAxis = document.getElementById("spectrum-freq-axis");
 const spectrumTooltip = document.getElementById("spectrum-tooltip");
 let spectrumSource = null;
 let spectrumReconnectTimer = null;
-let lastSpectrumData  = null;
 let spectrumDrawPending = false;
 let spectrumAxisKey = "";