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>

routes/tscm/sweep.py

@@ -19,12 +19,9 @@ from flask import Response, jsonify, request
 from data.tscm_frequencies import get_all_sweep_presets, get_sweep_preset
 from routes.tscm import (
     _baseline_recorder,
-    _current_sweep_id,
     _emit_event,
     _start_sweep_internal,
-    _sweep_running,
     tscm_bp,
-    tscm_queue,
 )
 from utils.database import get_tscm_sweep, update_tscm_sweep
 from utils.event_pipeline import process_event