[feat](trx-frontend-http): add auto-squelch button to Audio panel

Add an "Auto" button next to the SQL slider that sets the squelch
threshold to the current noise floor (estimated from spectrum bins)
plus a 6 dB margin. Uses the existing estimateNoiseFloorDb() heuristic.

https://claude.ai/code/session_01TDQyrZiPKfWGATVWPsLmHT
Signed-off-by: Claude <noreply@anthropic.com>

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

@@ -7669,6 +7669,29 @@ if (sdrSquelchEl) {
   });
 }
 
+const sdrSquelchAutoBtn = document.getElementById("sdr-squelch-auto");
+if (sdrSquelchAutoBtn) {
+  sdrSquelchAutoBtn.addEventListener("click", () => {
+    if (!sdrSquelchSupported) return;
+    const data = lastSpectrumData || window.lastSpectrumData;
+    if (!data || !Array.isArray(data.bins) || data.bins.length === 0) return;
+    const noiseDb = estimateNoiseFloorDb(data.bins);
+    if (noiseDb == null || !Number.isFinite(noiseDb)) return;
+    // Set threshold slightly above noise floor so squelch closes on noise
+    const thresholdDb = noiseDb + 6;
+    const clamped = Math.max(SDR_SQUELCH_MIN_DB, Math.min(SDR_SQUELCH_MAX_DB, thresholdDb));
+    const pct = clampSdrSquelchPercent(
+      ((clamped - SDR_SQUELCH_MIN_DB) / (SDR_SQUELCH_MAX_DB - SDR_SQUELCH_MIN_DB)) * 100,
+    );
+    if (sdrSquelchEl) {
+      sdrSquelchEl.value = String(pct);
+      updateSdrSquelchPctLabel();
+      saveSetting("sdrSquelchPct", pct);
+    }
+    submitSdrSquelchPercent(pct);
+  });
+}
+
 if (wfmAudioModeEl) {
   wfmAudioModeEl.value = loadSetting("wfmAudioMode", "stereo");
   wfmAudioModeEl.addEventListener("change", () => {