Favourites are fuel-scoped: starring pins the station for one fuel only

Fixing the bug where favouriting an Unleaded entry also created a Diesel
favourite. Favourites are now [FavouriteEntry] = (station, fuel) pairs:
- Starring a row pins it only for the fuel being viewed
- Favourites tabs show only fuels that actually have favourites
- Geofence monitor gives priority slots only to favourites matching the
  monitored fuel
- Tests updated + new fuel-scoping test (31/31 passing)

FuelBoard/ContentView.swift

@@ -10,7 +10,7 @@ struct ContentView: View {
     @State private var sortMode: SortMode = FuelStore.loadSortMode()
     @State private var stationLimit: Int = FuelStore.loadStationLimit()
     @State private var distanceUnit: DistanceUnit = FuelStore.loadDistanceUnit()
-    @State private var favourites: [FuelStation] = FuelStore.loadFavourites()
+    @State private var favourites: [FavouriteEntry] = FuelStore.loadFavourites()
     @State private var alertsEnabled: Bool = FuelStore.loadAlertsEnabled()
     @State private var alertsRadius: Double = FuelStore.loadAlertsRadius()
     @State private var location: Coordinate? = {
@@ -95,11 +95,13 @@ struct ContentView: View {
         }
     }
 
+    /// Station IDs favourited for the CURRENTLY selected fuel — the star on a
+    /// Stations-tab row reflects the fuel being viewed (fuel-scoped favourites).
     private var favouriteIDs: Set<String> {
-        Set(favourites.map(\.id))
+        Set(favourites.filter { $0.fuel == selectedFuel }.map(\.station.id))
     }
 
-    private var refreshedFavourites: [FuelStation] {
+    private var refreshedFavourites: [FavouriteEntry] {
         FuelStore.refreshedFavourites(favourites, from: stations)
     }
 
@@ -127,7 +129,6 @@ struct ContentView: View {
                 selectedFuel: selectedFuel,
                 location: location,
                 distanceUnit: distanceUnit,
-                favouriteIDs: favouriteIDs,
                 onToggleFavourite: toggleFavourite
             )
             .tabItem { Label("Favourites", systemImage: "star.fill") }
@@ -222,11 +223,12 @@ struct ContentView: View {
         }
     }
 
-    private func toggleFavourite(_ station: FuelStation) {
-        if favouriteIDs.contains(station.id) {
-            favourites.removeAll { $0.id == station.id }
+    private func toggleFavourite(_ station: FuelStation, fuel: FuelType) {
+        let key = FavouriteEntry(station: station, fuel: fuel)
+        if favourites.contains(where: { $0.id == key.id }) {
+            favourites.removeAll { $0.id == key.id }
         } else {
-            favourites.append(station)
+            favourites.append(key)
         }
         FuelStore.saveFavourites(favourites)
         WidgetCenter.shared.reloadAllTimelines()