Add GPS dongle support and fix Python 3.7/3.8 compatibility

- Add GPS dongle support with NMEA parsing (utils/gps.py, routes/gps.py)
- Add GPS device selector to ADS-B and Satellite observer location sections
- Add GPS dongle option to ADS-B dashboard
- Fix Python 3.7/3.8 compatibility by adding 'from __future__ import annotations'
  to all SDR module files (fixes TypeError: 'type' object is not subscriptable)
- Add pyserial to requirements.txt
- Update README with GPS dongle documentation and troubleshooting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

utils/sdr/detection.py

@@ -4,6 +4,8 @@ Multi-hardware SDR device detection.
 Detects RTL-SDR devices via rtl_test and other SDR hardware via SoapySDR.
 """
 
+from __future__ import annotations
+
 import logging
 import re
 import shutil