fix(adsb): disable bias-T on stop and warn when toggled while running (#207)

* fix(adsb): disable bias-T on stop and warn when toggled while running

The RTL-SDR bias-T hardware register persists after the device is closed,
so toggling bias-T off in the UI and stopping the SDR had no effect on the
actual hardware — verified with a multimeter in issue #205.

- Add disable_bias_t_via_rtl_biast() to rtlsdr.py (mirrors enable, uses -b 0)
- Track adsb_bias_t_active in adsb.py; call disable on stop_adsb() so the
  hardware register is cleared when ADS-B is stopped
- Show an inline warning in the UI when the bias-T checkbox is toggled while
  any SDR mode is active, since the setting only takes effect at start time

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

* fix(lint): remove unused imports in tscm sweep.py

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

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/index.html

@@ -6349,6 +6349,14 @@
         function saveBiasTSetting() {
             const enabled = document.getElementById('biasT')?.checked || false;
             localStorage.setItem('biasTEnabled', enabled);
+            // Warn if any SDR mode is currently running — bias-T is applied at
+            // start time and cannot be toggled on a running device.
+            const anyRunning = isRunning || isSensorRunning
+                || (typeof isAdsbRunning !== 'undefined' && isAdsbRunning)
+                || (typeof isAisRunning !== 'undefined' && isAisRunning);
+            if (anyRunning) {
+                showInfo('Bias-T change will take effect after restarting the active SDR mode');
+            }
         }
 
         function getBiasTEnabled() {