Alerts choose their own fuel, independent of the Stations tab

The proximity monitor was tied to the Stations-tab fuel selection, so
switching the list to another fuel silently re-targeted geofences. Now the
Alerts tab has its own 'Fuel to monitor' picker (persisted as
fuelboard.alertsFuel):
- Monitor reads loadAlertsFuel() on init/background relaunch
- Changing the alerts fuel re-registers geofences around stations selling
  that fuel only
- Favourites priority slots stay scoped to the monitored fuel
- Tests: alerts fuel round-trip + default (33/33 passing)

FuelBoard/ProximityMonitor.swift

@@ -28,7 +28,7 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         // Restore persisted state on background relaunch so region events work
         // even before the view fully appears.
         enabled = FuelStore.loadAlertsEnabled()
-        fuel = FuelStore.loadSelectedFuel()
+        fuel = FuelStore.loadAlertsFuel()
         radiusKM = FuelStore.loadAlertsRadius()
         stations = FuelStore.loadStations()
         favourites = FuelStore.loadFavourites().filter { $0.fuel == fuel }.map(\.station)