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

utils/sdr/base.py

@@ -5,6 +5,8 @@ This module provides the core abstractions for supporting multiple SDR hardware
 types (RTL-SDR, LimeSDR, HackRF, etc.) through a unified interface.
 """
 
+from __future__ import annotations
+
 from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from enum import Enum

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

utils/sdr/hackrf.py

@@ -5,6 +5,8 @@ Uses SoapySDR-based tools for FM demodulation and signal capture.
 HackRF supports 1 MHz to 6 GHz frequency range.
 """
 
+from __future__ import annotations
+
 from typing import Optional
 
 from .base import CommandBuilder, SDRCapabilities, SDRDevice, SDRType

utils/sdr/limesdr.py

@@ -5,6 +5,8 @@ Uses SoapySDR-based tools for FM demodulation and signal capture.
 LimeSDR supports 100 kHz to 3.8 GHz frequency range.
 """
 
+from __future__ import annotations
+
 from typing import Optional
 
 from .base import CommandBuilder, SDRCapabilities, SDRDevice, SDRType

utils/sdr/rtlsdr.py

@@ -5,6 +5,8 @@ Uses native rtl_* tools (rtl_fm, rtl_433) and dump1090 for maximum compatibility
 with existing RTL-SDR installations. No SoapySDR dependency required.
 """
 
+from __future__ import annotations
+
 from typing import Optional
 
 from .base import CommandBuilder, SDRCapabilities, SDRDevice, SDRType