feat: Add real-time Doppler tracking for ISS SSTV reception

- Add DopplerTracker class using skyfield for satellite tracking
- Calculate and apply Doppler shift correction (up to ±3.5 kHz at 145.800 MHz)
- Background thread monitors shift and retunes rtl_fm when >500 Hz drift
- New /sstv/doppler endpoint for real-time Doppler info
- Start endpoint accepts latitude/longitude for automatic tracking

Also:
- Add slowrx installation to setup.sh (source build for macOS, apt for Debian)
- Sync observer location to dashboard-specific localStorage keys

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

static/js/core/settings-manager.js

@@ -565,6 +565,17 @@ function loadObserverLocation() {
     if (currentLonDisplay) {
         currentLonDisplay.textContent = lon ? parseFloat(lon).toFixed(4) + '°' : 'Not set';
     }
+
+    // Sync dashboard-specific location keys for backward compatibility
+    if (lat && lon) {
+        const locationObj = JSON.stringify({ lat: parseFloat(lat), lon: parseFloat(lon) });
+        if (!localStorage.getItem('observerLocation')) {
+            localStorage.setItem('observerLocation', locationObj);
+        }
+        if (!localStorage.getItem('ais_observerLocation')) {
+            localStorage.setItem('ais_observerLocation', locationObj);
+        }
+    }
 }
 
 /**
@@ -650,6 +661,11 @@ function saveObserverLocation() {
     localStorage.setItem('observerLat', lat.toString());
     localStorage.setItem('observerLon', lon.toString());
 
+    // Also update dashboard-specific location keys for ADS-B and AIS
+    const locationObj = JSON.stringify({ lat: lat, lon: lon });
+    localStorage.setItem('observerLocation', locationObj);      // ADS-B dashboard
+    localStorage.setItem('ais_observerLocation', locationObj);  // AIS dashboard
+
     // Update display
     const currentLatDisplay = document.getElementById('currentLatDisplay');
     const currentLonDisplay = document.getElementById('currentLonDisplay');