feat(trx-client,http-frontend): spectrum waveform with frequency picker

Poll GetSpectrum every 200 ms in remote_client via a dedicated timer that
bypasses the main state-watch channel (no SSE noise). The resulting
SpectrumData is stored in FrontendRuntimeContext::spectrum and served by
a new GET /spectrum endpoint (JSON or 204 when unavailable).

HTTP frontend shows a spectrum panel (canvas + frequency axis) only when
the rig reports filter_controls=true (i.e. SoapySDR). The canvas renders:
- dark background with dBFS grid lines
- green FFT spectrum line with semi-transparent fill
- red dashed vertical marker at the currently tuned frequency
- frequency axis labels (MHz/kHz) below the canvas

Clicking the canvas tunes the rig to the clicked frequency.

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/index.html

@@ -158,6 +158,13 @@
           </label>
         </div>
       </div>
+      <div id="spectrum-panel" style="display:none;">
+        <div class="label"><span>Spectrum</span></div>
+        <div class="spectrum-wrap">
+          <canvas id="spectrum-canvas"></canvas>
+          <div id="spectrum-freq-axis"></div>
+        </div>
+      </div>
     </div>
     <div class="full-row label-below-row" id="audio-row">
       <div class="label"><span>Audio</span></div>