When navigating from another mode (e.g. pager) to the ADS-B dashboard,
the old process could still hold the USB device. Two fixes:
1. routes/adsb.py: If dump1090 starts but SBS port never comes up,
kill the process and return a DEVICE_BUSY error instead of silently
claiming success with no data.
2. templates/adsb_dashboard.html: Pre-flight conflict check in
toggleTracking() queries /devices/status and auto-stops any
conflicting mode before starting ADS-B, with a 1.5s USB release
delay.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>