Improve waterfall error handling and SDR tool path resolution

- Add pre-flight check for I/Q capture binary before spawning process
- Capture stderr from I/Q process for better error diagnostics
- Sync effective span value back to UI when backend adjusts it
- Use get_tool_path('rx_sdr') in Airspy, HackRF, LimeSDR, and SDRPlay
  command builders to support custom install locations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/js/modes/waterfall.js

@@ -2515,6 +2515,10 @@ const Waterfall = (function () {
                         _endMhz = msg.end_freq;
                         _drawFreqAxis();
                     }
+                    if (Number.isFinite(msg.effective_span_mhz)) {
+                        const spanEl = document.getElementById('wfSpanMhz');
+                        if (spanEl) spanEl.value = msg.effective_span_mhz;
+                    }
                     _setStatus(`Streaming ${_startMhz.toFixed(4)} - ${_endMhz.toFixed(4)} MHz`);
                     _setVisualStatus('RUNNING');
                     if (_monitoring) {