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/ContentView.swift

@@ -13,6 +13,7 @@ struct ContentView: View {
     @State private var favourites: [FavouriteEntry] = FuelStore.loadFavourites()
     @State private var alertsEnabled: Bool = FuelStore.loadAlertsEnabled()
     @State private var alertsRadius: Double = FuelStore.loadAlertsRadius()
+    @State private var alertsFuel: FuelType = FuelStore.loadAlertsFuel()
     @State private var location: Coordinate? = {
         if let loc = FuelStore.loadLocation() { return Coordinate(lat: loc.lat, lng: loc.lng) }
         return nil
@@ -136,6 +137,7 @@ struct ContentView: View {
             AlertsView(
                 enabled: $alertsEnabled,
                 radius: $alertsRadius,
+                fuel: $alertsFuel,
                 distanceUnit: distanceUnit,
                 monitoredCount: monitor.monitoredStationIDs.count,
                 lastAlert: monitor.lastAlert
@@ -162,7 +164,7 @@ struct ContentView: View {
                 showOnboarding = true
             }
             monitor.update(stations: stations, favourites: refreshedFavourites,
-                           fuel: selectedFuel, radiusKM: alertsRadius)
+                           fuel: alertsFuel, radiusKM: alertsRadius)
             monitor.setEnabled(alertsEnabled)
             // Refresh only when the cache is stale (twice-a-day policy).
             Task { await refresh() }
@@ -178,7 +180,7 @@ struct ContentView: View {
             if newPhase == .active {
                 locationManager.startForegroundTracking()
                 monitor.update(stations: stations, favourites: refreshedFavourites,
-                               fuel: selectedFuel, radiusKM: alertsRadius)
+                               fuel: alertsFuel, radiusKM: alertsRadius)
                 // No network fetch on foreground — pull-to-refresh is the override.
             } else {
                 locationManager.stopForegroundTracking()
@@ -192,7 +194,7 @@ struct ContentView: View {
                 // Geofences follow the user's position, but the station list is
                 // NOT re-fetched on every movement (cached, twice-a-day policy).
                 monitor.update(stations: stations, favourites: refreshedFavourites,
-                               fuel: selectedFuel, radiusKM: alertsRadius)
+                               fuel: alertsFuel, radiusKM: alertsRadius)
             }
         }
         .onChange(of: selectedFuel) { _, _ in
@@ -211,7 +213,7 @@ struct ContentView: View {
             FuelStore.saveAlertsEnabled(newValue)
             monitor.setEnabled(newValue)
             monitor.update(stations: stations, favourites: refreshedFavourites,
-                           fuel: selectedFuel, radiusKM: alertsRadius)
+                           fuel: alertsFuel, radiusKM: alertsRadius)
             if newValue {
                 locationManager.startBackgroundTracking()
             }
@@ -219,7 +221,14 @@ struct ContentView: View {
         .onChange(of: alertsRadius) { _, newValue in
             FuelStore.saveAlertsRadius(newValue)
             monitor.update(stations: stations, favourites: refreshedFavourites,
-                           fuel: selectedFuel, radiusKM: alertsRadius)
+                           fuel: alertsFuel, radiusKM: alertsRadius)
+        }
+        .onChange(of: alertsFuel) { _, newValue in
+            // Alerts fuel is independent of the Stations-tab selection —
+            // changing it re-targets geofences to stations selling that fuel.
+            FuelStore.saveAlertsFuel(newValue)
+            monitor.update(stations: stations, favourites: refreshedFavourites,
+                           fuel: newValue, radiusKM: alertsRadius)
         }
     }
 
@@ -233,7 +242,7 @@ struct ContentView: View {
         FuelStore.saveFavourites(favourites)
         WidgetCenter.shared.reloadAllTimelines()
         monitor.update(stations: stations, favourites: refreshedFavourites,
-                       fuel: selectedFuel, radiusKM: alertsRadius)
+                       fuel: alertsFuel, radiusKM: alertsRadius)
     }
 
     /// Fetches fresh prices, but only when the cache is stale — unless
@@ -261,7 +270,7 @@ struct ContentView: View {
         }
         // Keep monitor geofences in sync with the freshest data.
         monitor.update(stations: stations, favourites: refreshedFavourites,
-                       fuel: selectedFuel, radiusKM: alertsRadius)
+                       fuel: alertsFuel, radiusKM: alertsRadius)
     }
 }