Add Airspy SDR support and persist GPS coordinates

Airspy support:
- Add AIRSPY to SDRType enum and driver mappings
- Create AirspyCommandBuilder using SoapySDR tools (rx_fm, readsb, rtl_433)
- Register in SDRFactory and add to hardware type dropdown
- Supports Airspy R2/Mini (24MHz-1.8GHz) and HF+ devices

GPS coordinate persistence:
- Save observer location to localStorage when manually entered or via geolocation
- Restore saved coordinates on page load in both index.html and adsb_dashboard.html
- Coordinates are shared between both pages

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

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

utils/sdr/base.py

@@ -18,6 +18,7 @@ class SDRType(Enum):
     RTL_SDR = "rtlsdr"
     LIME_SDR = "limesdr"
     HACKRF = "hackrf"
+    AIRSPY = "airspy"
     # Future support
     # USRP = "usrp"
     # BLADE_RF = "bladerf"