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/StationsView.swift

@@ -14,7 +14,7 @@ struct StationsView: View {
     let topStationID: String?
     let location: Coordinate?
     let favouriteIDs: Set<String>
-    var onToggleFavourite: (FuelStation) -> Void = { _ in }
+    var onToggleFavourite: (FuelStation, FuelType) -> Void = { _, _ in }
     var onRefresh: () async -> Void = {}
 
     /// Pagination: one page = 10 rows, reset whenever the underlying list
@@ -107,7 +107,7 @@ struct StationsView: View {
                                 baselinePrice: baselinePrice,
                                 isTopResult: station.id == topStationID,
                                 isFavourite: favouriteIDs.contains(station.id),
-                                onToggleFavourite: { onToggleFavourite(station) }
+                                onToggleFavourite: { onToggleFavourite(station, selectedFuel) }
                             )
                         }