[fix](trx-frontend): gate RDS display updates on WFM mode

updateRdsPsOverlay was called on every spectrum frame (25 Hz)
regardless of mode, doing 15+ DOM element lookups and text updates
even in USB/AM/CW/etc.  The server-side RDS DSP already only runs
in WFM; align the client:

- Spectrum SSE handler: only increment rdsFrameCount and call
  updateRdsPsOverlay when lastModeName === "WFM"
- Mode change: call resetRdsDisplay() when switching to or from WFM
  so the overlay and RDS panel are cleared promptly

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

@@ -1444,6 +1444,9 @@ function render(update) {
     modeEl.value = modeUpper;
     if (modeUpper === "WFM" && lastModeName !== "WFM") {
       setJogDivisor(10);
+      resetRdsDisplay();
+    } else if (modeUpper !== "WFM" && lastModeName === "WFM") {
+      resetRdsDisplay();
     }
     lastModeName = modeUpper;
     updateWfmControls();
@@ -3692,11 +3695,13 @@ function startSpectrumStreaming() {
     try {
       lastSpectrumData = JSON.parse(evt.data);
       lastSpectrumRenderData = buildSpectrumRenderData(lastSpectrumData);
-      rdsFrameCount++;
       pushOverviewWaterfallFrame(lastSpectrumData);
       refreshCenterFreqDisplay();
       scheduleSpectrumDraw();
-      updateRdsPsOverlay(lastSpectrumData.rds);
+      if (lastModeName === "WFM") {
+        rdsFrameCount++;
+        updateRdsPsOverlay(lastSpectrumData.rds);
+      }
     } catch (_) {}
   };
   spectrumSource.onerror = () => {