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/FavouritesView.swift

@@ -15,6 +15,16 @@ struct FavouritesView: View {
     let distanceUnit: DistanceUnit
     let priceDisplayStyle: PriceDisplayStyle
     let stationTapAction: StationTapAction
+    /// Fuel Savings config. When it is configured this tab shows the SAME
+    /// fill-up control and per-row estimate as the Stations list: one session
+    /// amount is shared, so adjusting it in either list updates both, and it
+    /// still never touches the Settings default.
+    let fuelSavingsSettings: FuelSavingsSettings
+    @Binding var fuelSavingsAmount: FuelSavingsFillAmount
+    /// Estimated saving for a favourite, against the nearest eligible station —
+    /// the same baseline and definition the Stations list uses.
+    var savingsForStation: (FuelStation) -> FuelSavingsAvailability? = { _ in nil }
+    var savingsBaselineStationName: String?
     var onToggleFavourite: (FuelStation, FuelType) -> Void = { _, _ in }
     /// Persists a reordered favourites array (after drag-and-drop).
     var onReorder: ([FavouriteEntry]) -> Void = { _ in }
@@ -70,6 +80,10 @@ struct FavouritesView: View {
          distanceUnit: DistanceUnit,
          priceDisplayStyle: PriceDisplayStyle,
          stationTapAction: StationTapAction,
+         fuelSavingsSettings: FuelSavingsSettings = .defaults,
+         fuelSavingsAmount: Binding<FuelSavingsFillAmount>,
+         savingsForStation: @escaping (FuelStation) -> FuelSavingsAvailability? = { _ in nil },
+         savingsBaselineStationName: String? = nil,
          onToggleFavourite: @escaping (FuelStation, FuelType) -> Void = { _, _ in },
          onReorder: @escaping ([FavouriteEntry]) -> Void = { _ in },
          onHistoryUnavailable: (() -> Void)? = nil,
@@ -80,6 +94,10 @@ struct FavouritesView: View {
         self.distanceUnit = distanceUnit
         self.priceDisplayStyle = priceDisplayStyle
         self.stationTapAction = stationTapAction
+        self.fuelSavingsSettings = fuelSavingsSettings
+        _fuelSavingsAmount = fuelSavingsAmount
+        self.savingsForStation = savingsForStation
+        self.savingsBaselineStationName = savingsBaselineStationName
         self.onToggleFavourite = onToggleFavourite
         self.onReorder = onReorder
         self.onHistoryUnavailable = onHistoryUnavailable
@@ -124,6 +142,17 @@ struct FavouritesView: View {
                         )
                     }
 
+                    if fuelSavingsSettings.isConfigured {
+                        Section {
+                            FuelSavingsAdjustmentView(
+                                amount: $fuelSavingsAmount,
+                                referencePricePencePerLitre: cheapestPrice
+                            )
+                        } footer: {
+                            Text(String(localized: "Savings below are estimated against the nearest station with a price for this fuel — the same baseline as the Stations list.", comment: "Favourites fuel-savings explanation"))
+                        }
+                    }
+
                     Section {
                         ForEach(Array(ordered.enumerated()), id: \.element.id) { index, station in
                             StationRow(
@@ -133,9 +162,11 @@ struct FavouritesView: View {
                                 distanceUnit: distanceUnit,
                                 priceDisplayStyle: priceDisplayStyle,
                                 baselinePrice: cheapestPrice,
-                                isTopResult: index == 0,
+                                isTopResult: station.id == cheapest?.id,
                                 isFavourite: activeFuelFavouriteIDs.contains(station.id),
+                                topBadgeText: "CHEAPEST",
                                 tapAction: stationTapAction,
+                                fuelSavings: savingsForStation(station),
                                 onToggleFavourite: { onToggleFavourite(station, activeFuel) },
                                 onShowDetails: { detailStation = $0 }
                             )
@@ -183,7 +214,10 @@ struct FavouritesView: View {
                 StationDetailView(
                     station: station,
                     location: location,
-                    distanceUnit: distanceUnit
+                    distanceUnit: distanceUnit,
+                    fuelSavings: savingsForStation(station),
+                    fuelSavingsAmount: fuelSavingsAmount,
+                    savingsBaselineStationName: savingsBaselineStationName
                 )
             }
             .onAppear {