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)

Shared/FuelStore.swift

@@ -247,6 +247,7 @@ struct FuelStore {
     static let favouritesKey = "fuelboard.favourites"      // [FavouriteEntry] JSON
     static let alertsEnabledKey = "fuelboard.alertsEnabled" // Bool
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
+    static let alertsFuelKey = "fuelboard.alertsFuel"       // FuelType raw value
     static let onboardingCompletedKey = "fuelboard.onboardingCompleted" // Bool
     static let lastRefreshKey = "fuelboard.lastRefresh"      // TimeInterval (seconds since 1970)
 
@@ -312,6 +313,21 @@ struct FuelStore {
         saveString(fuel.rawValue, service: fuelKey)
     }
 
+    // MARK: Alerts fuel
+
+    /// The fuel proximity alerts monitor — independent of the Stations-tab
+    /// selection so users can browse any fuel without re-targeting alerts.
+    static func loadAlertsFuel() -> FuelType {
+        if let raw = loadString(service: alertsFuelKey), let fuel = FuelType(rawValue: raw) {
+            return fuel
+        }
+        return .e10
+    }
+
+    static func saveAlertsFuel(_ fuel: FuelType) {
+        saveString(fuel.rawValue, service: alertsFuelKey)
+    }
+
     // MARK: Sort mode
 
     static func loadSortMode() -> SortMode {