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/__init__.py

@@ -21,6 +21,8 @@ Example usage:
     cmd = builder.build_fm_demod_command(device, frequency_mhz=153.35)
 """
 
+from __future__ import annotations
+
 from typing import Optional
 
 from .base import CommandBuilder, SDRCapabilities, SDRDevice, SDRType