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

Shared/FuelStore.swift

@@ -48,21 +48,75 @@ enum RAGRating: Int, Codable {
 }
 
 enum FuelType: String, Codable, CaseIterable, Identifiable {
-    case e10    // Unleaded 95 (E10)
-    case e5     // Premium 97/98 (E5)
+    case e10    // Unleaded (E10)
+    case e5     // Premium (E5)
     case diesel // B7 diesel
 
     var id: String { rawValue }
 
     var displayName: 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"
         }
     }
 }
 
+/// Display unit for all distances in the app + widget. Internally distances
+/// are always stored/computed in km; conversion happens at the display and
+/// filter boundary so nothing else needs to know the unit.
+enum DistanceUnit: String, Codable, CaseIterable, Identifiable {
+    case miles
+    case kilometers
+
+    var id: String { rawValue }
+
+    var displayName: String {
+        switch self {
+        case .miles: return "Miles"
+        case .kilometers: return "Kilometres"
+        }
+    }
+
+    /// Short suffix for values ("5 mi", "3.2 km").
+    var shortName: String {
+        switch self {
+        case .miles: return "mi"
+        case .kilometers: return "km"
+        }
+    }
+
+    /// Full word for narrative text ("within 5 miles", "within 8 km").
+    var label: String {
+        switch self {
+        case .miles: return "miles"
+        case .kilometers: return "km"
+        }
+    }
+
+    /// Convert a value expressed in this unit to km.
+    func toKM(_ value: Double) -> Double {
+        switch self {
+        case .miles: return value * 1.60934
+        case .kilometers: return value
+        }
+    }
+
+    /// Convert a km value to this unit.
+    func fromKM(_ km: Double) -> Double {
+        switch self {
+        case .miles: return km * 0.621371
+        case .kilometers: return km
+        }
+    }
+
+    /// Format a km distance in this unit ("1.2 mi", "3.4 km").
+    func format(_ km: Double) -> String {
+        String(format: "%.1f %@", fromKM(km), shortName)
+    }
+}
+
 // MARK: - Station model
 
 struct FuelStation: Identifiable, Codable, Equatable {
@@ -179,6 +233,7 @@ struct FuelStore {
     static let fuelKey = "fuelboard.selectedFuel"      // FuelType raw value
     static let sortModeKey = "fuelboard.sortMode"      // SortMode raw value
     static let stationLimitKey = "fuelboard.stationLimitMiles" // Int miles (5/10/15)
+    static let distanceUnitKey = "fuelboard.distanceUnit"      // DistanceUnit raw value
     static let favouritesKey = "fuelboard.favourites"      // [FuelStation] JSON
     static let alertsEnabledKey = "fuelboard.alertsEnabled" // Bool
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
@@ -277,6 +332,20 @@ struct FuelStore {
         saveString(String(miles), service: stationLimitKey)
     }
 
+    // MARK: Distance unit — miles or kilometres. Stored raw value; default
+    // miles for backward compatibility with pre-toggle installs.
+
+    static func loadDistanceUnit() -> DistanceUnit {
+        if let raw = loadString(service: distanceUnitKey), let unit = DistanceUnit(rawValue: raw) {
+            return unit
+        }
+        return .miles
+    }
+
+    static func saveDistanceUnit(_ unit: DistanceUnit) {
+        saveString(unit.rawValue, service: distanceUnitKey)
+    }
+
     // MARK: Favourites
 
     static func loadFavourites() -> [FuelStation] {