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/ContentView.swift

@@ -9,6 +9,7 @@ struct ContentView: View {
     @State private var selectedFuel: FuelType = FuelStore.loadSelectedFuel()
     @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 alertsEnabled: Bool = FuelStore.loadAlertsEnabled()
     @State private var alertsRadius: Double = FuelStore.loadAlertsRadius()
@@ -31,7 +32,7 @@ struct ContentView: View {
         let selling = stations.filter { $0.prices[selectedFuel] != nil }
         guard let location else { return selling }
         if sortMode == .closest { return selling }  // radius disabled in Closest
-        let radiusKM = Double(stationLimit) * 1.60934  // chosen miles → km
+        let radiusKM = distanceUnit.toKM(Double(stationLimit))  // chosen units → km
         return selling.filter {
             $0.distanceKM(to: location.lat, lng2: location.lng) <= radiusKM
         }
@@ -47,7 +48,7 @@ struct ContentView: View {
     private var baselinePrice: Double? {
         let pool = poolStations
         if sortMode == .closest, let location {
-            let radiusKM = Double(stationLimit) * 1.60934  // chosen miles → km
+            let radiusKM = distanceUnit.toKM(Double(stationLimit))  // chosen units → km
             let within = pool.filter {
                 $0.distanceKM(to: location.lat, lng2: location.lng) <= radiusKM
             }
@@ -111,6 +112,7 @@ struct ContentView: View {
                 selectedFuel: $selectedFuel,
                 sortMode: $sortMode,
                 stationLimit: $stationLimit,
+                distanceUnit: distanceUnit,
                 baselinePrice: baselinePrice,
                 topStationID: topStationID,
                 location: location,
@@ -124,6 +126,7 @@ struct ContentView: View {
                 favourites: refreshedFavourites,
                 selectedFuel: selectedFuel,
                 location: location,
+                distanceUnit: distanceUnit,
                 favouriteIDs: favouriteIDs,
                 onToggleFavourite: toggleFavourite
             )
@@ -132,11 +135,17 @@ struct ContentView: View {
             AlertsView(
                 enabled: $alertsEnabled,
                 radius: $alertsRadius,
+                distanceUnit: distanceUnit,
                 monitoredCount: monitor.monitoredStationIDs.count,
-                lastAlert: monitor.lastAlert,
-                onShowOnboarding: { showOnboarding = true }
+                lastAlert: monitor.lastAlert
             )
             .tabItem { Label("Alerts", systemImage: "bell.fill") }
+
+            SettingsView(
+                distanceUnit: $distanceUnit,
+                onShowOnboarding: { showOnboarding = true }
+            )
+            .tabItem { Label("Settings", systemImage: "gearshape.fill") }
         }
         .fullScreenCover(isPresented: $showOnboarding) {
             OnboardingView {
@@ -191,8 +200,9 @@ struct ContentView: View {
         }
         .onChange(of: stationLimit) { _, newValue in
             // Distance filter is LOCAL math now — the cache holds the full-UK
-            // dump, so changing 5/10/15 miles never needs a network fetch.
-            // sortedStations/radiusScopedStations recompute on the next render.
+            // dump, so changing 5/10/15 (miles or km) never needs a network
+            // fetch. sortedStations/radiusScopedStations recompute on the
+            // next render.
             FuelStore.saveStationLimit(newValue)
             WidgetCenter.shared.reloadAllTimelines()
         }
@@ -259,6 +269,7 @@ struct StationRow: View {
     let station: FuelStation
     let fuel: FuelType
     let location: Coordinate?
+    let distanceUnit: DistanceUnit
     let baselinePrice: Double?
     let isTopResult: Bool
     let isFavourite: Bool
@@ -325,7 +336,7 @@ struct StationRow: View {
                     .lineLimit(1)
                     .truncationMode(.tail)
                 if let location {
-                    Text(String(format: "%.1f mi", station.distanceKM(to: location.lat, lng2: location.lng) * 0.621371))
+                    Text(distanceUnit.format(station.distanceKM(to: location.lat, lng2: location.lng)))
                         .font(.caption2)
                         .foregroundStyle(.secondary)
                         .monospacedDigit()