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/ProximityMonitor.swift

@@ -155,14 +155,15 @@ final class ProximityMonitor: NSObject, ObservableObject, @preconcurrency CLLoca
 
     private func fireAlert(for station: FuelStation, price: Double) {
         let brand = station.brand.isEmpty ? station.name : station.brand
+        let unit = FuelStore.loadDistanceUnit()
         let content = UNMutableNotificationContent()
         content.title = "Cheapest \(fuel.displayName) nearby: \(brand)"
-        content.body = "\(station.name) is the cheapest within \(Int(radiusKM)) km at \(String(format: "%.1fp", price)). Tap to open."
+        content.body = "\(station.name) is the cheapest within \(unit.format(radiusKM)) at \(String(format: "%.1fp", price)). Tap to open."
         content.sound = .default
 
         let request = UNNotificationRequest(identifier: UUID().uuidString, content: content, trigger: nil)
         UNUserNotificationCenter.current().add(request)
 
-        lastAlert = "\(brand) · \(String(format: "%.1fp", price)) · \(Int(radiusKM)) km radius"
+        lastAlert = "\(brand) · \(String(format: "%.1fp", price)) · \(unit.format(radiusKM)) radius"
     }
 }