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>
sjg
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