[fix](trx-frontend): disable marker overlay for non-sdr

Collapse the shared signal marker overlay when no spectrum data is available so the non-SDR signal graph renders cleanly.

Co-authored-by: OpenAI Codex <codex@openai.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

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

@@ -710,6 +710,12 @@ function signalOverlayHeight() {
 
 function drawSignalOverlay() {
   if (!signalOverlayCanvas || !signalVisualBlockEl) return;
+  if (!lastSpectrumData) {
+    signalOverlayCanvas.style.height = "0";
+    signalOverlayCanvas.width = 0;
+    signalOverlayCanvas.height = 0;
+    return;
+  }
   const cssW = Math.floor(signalVisualBlockEl.clientWidth);
   const cssH = signalOverlayHeight();
   signalOverlayCanvas.style.height = cssH > 0 ? `${cssH}px` : "0";
@@ -734,7 +740,6 @@ function drawSignalOverlay() {
   ctx.scale(dpr, dpr);
   ctx.clearRect(0, 0, cssW, cssH);
 
-  if (lastSpectrumData) {
   const range = spectrumVisibleRange(lastSpectrumData);
   const hzToX = (hz) => ((hz - range.visLoHz) / range.visSpanHz) * cssW;
 
@@ -779,7 +784,6 @@ function drawSignalOverlay() {
       ctx.restore();
     }
   }
-  }
 
   ctx.restore();
 }