Add Worth the Drive fuel savings with road-routed estimates

Optional Fuel Savings feature (off by default): vehicle MPG in UK imperial gallons, fill amount in pounds or litres, persisted through the existing keychain/app-group store. Ranks the WHOLE station pool by estimated net saving (fuel-price saving at each pump minus the extra return-trip driving cost) using MapKit road distances with a straight-line fallback marked (est.).

The closest station with a valid price for the selected fuel is the comparison baseline and is labelled "Closest station"; other rows read "Save £X.XX after travel" / "£X.XX worse off after travel" / "Break even". Labels live in the shared model so they are unit-tested. The TOP badge still marks the cheapest pump price and reads "CHEAPEST" when the list is ranked by net saving.

Road routing now prioritises the Worth-the-Drive candidates plus the baseline and merges cache entries instead of replacing them, so far-out stations get real routes rather than permanent straight-line estimates. Numeric fields gain a Done button above the keypad (decimalPad has no Return key); the fill-up control is hidden unless the feature is configured; Favourites shares one session fill amount with the station list. 131 tests.

FuelBoard/StationDetailView.swift

@@ -10,6 +10,9 @@ struct StationDetailView: View {
     let station: FuelStation
     let location: Coordinate?
     let distanceUnit: DistanceUnit
+    var fuelSavings: FuelSavingsAvailability? = nil
+    var fuelSavingsAmount: FuelSavingsFillAmount = FuelSavingsSettings.defaultPreferredAmount
+    var savingsBaselineStationName: String? = nil
     @Environment(\.dismiss) private var dismiss
     @State private var showingReportIssueConfirmation = false
 
@@ -54,6 +57,16 @@ struct StationDetailView: View {
                             .background(Capsule().fill(Color.orange.opacity(0.14)))
                     }
                 }
+                Section {
+                    FuelSavingsSummaryView(
+                        availability: fuelSavings,
+                        amount: fuelSavingsAmount,
+                        baselineStationName: savingsBaselineStationName
+                    )
+                } header: {
+                    Text("Worth the drive?")
+                }
+
                 Section {
                     Button {
                         if let url = station.mapsDirectionsURL {