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/ProximityMonitor.swift

@@ -31,15 +31,20 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         fuel = FuelStore.loadSelectedFuel()
         radiusKM = FuelStore.loadAlertsRadius()
         stations = FuelStore.loadStations()
-        favourites = FuelStore.loadFavourites()
+        favourites = FuelStore.loadFavourites().filter { $0.fuel == fuel }.map(\.station)
     }
 
     /// Re-registers geofences. Call whenever stations/favourites/settings change.
-    func update(stations: [FuelStation], favourites: [FuelStation], fuel: FuelType, radiusKM: Double) {
+    /// Favourites are fuel-scoped entries; only entries for the monitored fuel
+    /// get priority slots (a Diesel favourite must not grab a slot while
+    /// monitoring Unleaded).
+    func update(stations: [FuelStation], favourites: [FavouriteEntry], fuel: FuelType, radiusKM: Double) {
         // Fall back to the shared cache when called before the first fetch
         // completes (launch, background region-event wake).
         self.stations = stations.isEmpty ? FuelStore.loadStations() : stations
-        self.favourites = favourites.isEmpty ? FuelStore.loadFavourites() : favourites
+        self.favourites = favourites.isEmpty
+            ? FuelStore.loadFavourites().filter { $0.fuel == fuel }.map(\.station)
+            : favourites.filter { $0.fuel == fuel }.map(\.station)
         self.fuel = fuel
         self.radiusKM = radiusKM
 
@@ -51,8 +56,9 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         guard enabled else { return }
 
         // Favourites first (guaranteed slots), then closest stations, max 18.
-        var candidates: [FuelStation] = favourites
-        let favIDs = Set(favourites.map(\.id))
+        // `self.favourites` is already filtered to the monitored fuel.
+        var candidates: [FuelStation] = self.favourites
+        let favIDs = Set(self.favourites.map(\.id))
         let location = FuelStore.loadLocation()
         let others = stations
             .filter { !favIDs.contains($0.id) }
@@ -88,7 +94,7 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
         } else {
             requestPermissions()
             // Re-register geofences from restored cache immediately.
-            update(stations: stations, favourites: favourites, fuel: fuel, radiusKM: radiusKM)
+            update(stations: stations, favourites: FuelStore.loadFavourites(), fuel: fuel, radiusKM: radiusKM)
         }
     }