Add Settings tab: distance units toggle, onboarding replay, £4.99 tip; drop E10/E5 from labels

- FuelType labels: 'Unleaded (E10)'/'Premium (E5)' -> 'Unleaded'/'Premium'
- New DistanceUnit (miles/km) preference; all app + widget distances,
  radii and alert text convert/display in the chosen unit
- Settings tab (4th tab): segmented miles/km picker, 'Show introduction'
  replay button (onboarding moved out of Alerts tab), StoreKit tip £4.99
- ProximityMonitor + widget notifications use the chosen unit
- Tests: label expectations updated, DistanceUnit conversion + format tests

FuelBoard/FavouritesView.swift

@@ -6,6 +6,7 @@ struct FavouritesView: View {
     let favourites: [FuelStation]
     let selectedFuel: FuelType
     let location: Coordinate?
+    let distanceUnit: DistanceUnit
     let favouriteIDs: Set<String>
     var onToggleFavourite: (FuelStation) -> Void = { _ in }
 
@@ -62,6 +63,7 @@ struct FavouritesView: View {
                                 station: station,
                                 fuel: selectedFuel,
                                 location: location,
+                                distanceUnit: distanceUnit,
                                 baselinePrice: cheapestPrice,
                                 isTopResult: index == 0,
                                 isFavourite: favouriteIDs.contains(station.id),