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/AlertsView.swift

@@ -4,10 +4,14 @@ import SwiftUI
 /// being monitored.
 struct AlertsView: View {
     @Binding var enabled: Bool
-    @Binding var radius: Double
+    @Binding var radius: Double   // stored in km (monitor + storage)
+    let distanceUnit: DistanceUnit
     let monitoredCount: Int
     let lastAlert: String?
-    var onShowOnboarding: () -> Void = {}
+
+    /// The radius slider works in the user's chosen unit; the stored value
+    /// stays km so ProximityMonitor and persistence never change.
+    private var radiusInUnit: Double { distanceUnit.fromKM(radius) }
 
     var body: some View {
         NavigationStack {
@@ -24,11 +28,15 @@ struct AlertsView: View {
                             HStack {
                                 Text("Radius")
                                 Spacer()
-                                Text("\(Int(radius)) km")
+                                Text(String(format: "%.1f %@", radiusInUnit, distanceUnit.shortName))
                                     .foregroundStyle(.secondary)
                                     .monospacedDigit()
                             }
-                            Slider(value: $radius, in: 1...10, step: 1)
+                            // 1–10 in the user's unit; convert back to km on change.
+                            Slider(value: Binding(
+                                get: { radiusInUnit },
+                                set: { radius = distanceUnit.toKM($0) }
+                            ), in: 1...10, step: distanceUnit == .kilometers ? 1 : 0.5)
                         }
                     }
 
@@ -42,7 +50,7 @@ struct AlertsView: View {
                             Text("Your favourites get priority, then the closest stations fill the rest (18 max, iOS region limit).")
                                 .font(.caption)
                                 .foregroundStyle(.secondary)
-                            Text("Alerts are checked against the cheapest station within \(Int(radius)) km for the selected fuel. Each station alerts at most once per hour.")
+                            Text("Alerts are checked against the cheapest station within the trigger radius for the selected fuel. Each station alerts at most once per hour.")
                                 .font(.caption)
                                 .foregroundStyle(.secondary)
                         }
@@ -55,19 +63,6 @@ struct AlertsView: View {
                         }
                     }
                 }
-
-                // Testing hook — in production onboarding shows once at first
-                // launch; this button re-opens it to verify the flow.
-                Section {
-                    Button {
-                        onShowOnboarding()
-                    } label: {
-                        Label("Show onboarding (testing)", systemImage: "flag.fill")
-                            .font(.footnote)
-                    }
-                } footer: {
-                    Text("Testing only — onboarding normally appears once on first launch.")
-                }
             }
             .navigationTitle("Alerts")
         }