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/StationsView.swift

@@ -9,6 +9,7 @@ struct StationsView: View {
     @Binding var selectedFuel: FuelType
     @Binding var sortMode: SortMode
     @Binding var stationLimit: Int
+    let distanceUnit: DistanceUnit
     let baselinePrice: Double?
     let topStationID: String?
     let location: Coordinate?
@@ -29,11 +30,11 @@ struct StationsView: View {
                 Section {
                     if let location {
                         if sortMode == .closest {
-                            Text("Closest \(selectedFuel.displayName) stations — nearest first, best value within \(stationLimit) miles.")
+                            Text("Closest \(selectedFuel.displayName) stations — nearest first, best value within \(stationLimit) \(distanceUnit.label).")
                                 .font(.footnote)
                                 .foregroundStyle(.secondary)
                         } else {
-                            Text("Cheapest \(selectedFuel.displayName) within \(stationLimit) miles — tap a station for directions.")
+                            Text("Cheapest \(selectedFuel.displayName) within \(stationLimit) \(distanceUnit.label) — tap a station for directions.")
                                 .font(.footnote)
                                 .foregroundStyle(.secondary)
                         }
@@ -60,7 +61,7 @@ struct StationsView: View {
                 Section("Distance") {
                     Picker("Distance", selection: $stationLimit) {
                         ForEach(FuelStore.stationRadiusOptions, id: \.self) { miles in
-                            Text("\(miles) miles").tag(miles)
+                            Text("\(miles) \(distanceUnit.label)").tag(miles)
                         }
                     }
                     .pickerStyle(.segmented)
@@ -79,7 +80,7 @@ struct StationsView: View {
                             .font(.caption2)
                             .foregroundStyle(.secondary)
                     } else {
-                        Text("\(totalCount) stations within \(stationLimit) miles")
+                        Text("\(totalCount) stations within \(stationLimit) \(distanceUnit.label)")
                             .font(.caption2)
                             .foregroundStyle(.secondary)
                     }
@@ -111,6 +112,7 @@ struct StationsView: View {
                                 station: station,
                                 fuel: selectedFuel,
                                 location: location,
+                                distanceUnit: distanceUnit,
                                 baselinePrice: baselinePrice,
                                 isTopResult: station.id == topStationID,
                                 isFavourite: favouriteIDs.contains(station.id),
@@ -193,12 +195,12 @@ struct StationsView: View {
 // MARK: - Fuel-type iconography (app target only — FuelStore.swift is Foundation-only)
 
 extension FuelType {
-    /// Short segment label ("Unleaded (E10)" / "Premium (E5)" / "Diesel") for
+    /// Short segment label ("Unleaded" / "Premium" / "Diesel") for
     /// the picker and description.
     var shortName: String {
         switch self {
-        case .e10: return "Unleaded (E10)"
-        case .e5: return "Premium (E5)"
+        case .e10: return "Unleaded"
+        case .e5: return "Premium"
         case .diesel: return "Diesel"
         }
     }