Add cheapest-favourite price-drop alerts

FuelBoard/AlertsView.swift

@@ -12,6 +12,8 @@ struct AlertsView: View {
     @Binding var enabled: Bool
     @Binding var radius: Double   // stored in km (monitor + storage)
     @Binding var fuel: FuelType   // the fuel alerts monitor for
+    @Binding var favouriteDropEnabled: Bool
+    @Binding var favouriteDropFuel: FuelType
     /// Whether alerts mirror the Stations-tab search distance (capped at 8 mi).
     @Binding var followsSearch: Bool
     /// Whether the "cheapest nearby" Live Activity is shown on the Lock Screen
@@ -157,6 +159,19 @@ struct AlertsView: View {
                     Text("When you approach a station that is the cheapest within the radius, FuelBoard sends a notification — even with the app closed. Nearby stations selling \(fuel.displayName.lowercased()) are watched — favourites get priority — and each station alerts at most once per hour. Follow search mirrors the Stations-tab distance, capped at 8 miles for reliable geofencing. Smaller radii ping more often but later; larger radii ping rarely in town but earlier from afar.")
                 }
 
+                Section {
+                    Toggle("Cheapest-favourite price-drop alerts", isOn: $favouriteDropEnabled)
+                    Picker("Fuel", selection: $favouriteDropFuel) {
+                        ForEach(FuelType.allCases) { fuel in
+                            Text(fuel.displayName).tag(fuel)
+                        }
+                    }
+                } header: {
+                    Text("Favourite price-drop alerts")
+                } footer: {
+                    Text("Watches your cheapest saved favourite for the fuel you pick here. FuelBoard alerts when a different favourite becomes the cheapest, or when your current cheapest favourite drops by at least 1.0p. This is separate from the nearby-geofence alert, so you can monitor favourites without extra location noise.")
+                }
+
                 if enabled, let lastAlert {
                     Section("Last alert") {
                         Label(lastAlert, systemImage: "bell.badge.fill")