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/SettingsView.swift

@@ -19,6 +19,7 @@ struct SettingsView: View {
     @Binding var distanceUnit: DistanceUnit
     @Binding var priceDisplayStyle: PriceDisplayStyle
     @Binding var stationTapAction: StationTapAction
+    @Binding var fuelSavingsSettings: FuelSavingsSettings
     @Binding var dataRefreshMode: DataRefreshMode
     /// The fuel + radius currently configured for alerts (mirrors the Alerts
     /// tab) so the test notification matches what real alerts will say.
@@ -70,6 +71,16 @@ struct SettingsView: View {
     @State private var mediumWidgetDiag: String?
     @State private var installedWidgets: String = ""
 
+    /// Which numeric field in the Fuel Savings section is being edited. Those
+    /// fields use `.decimalPad`, which has no Return key, so the keyboard's
+    /// Done button clears this to dismiss AND commit the typed value.
+    @FocusState private var focusedSavingsField: SavingsField?
+
+    private enum SavingsField: Hashable {
+        case averageMPG
+        case fillAmount
+    }
+
     private func refreshWidgetDiag() {
         smallWidgetDiag = FuelStore.loadWidgetDiag(intentType: "FuelBoardSmallWidget")
         mediumWidgetDiag = FuelStore.loadWidgetDiag(intentType: "FuelBoardWidgetConfigurationIntent")
@@ -137,6 +148,45 @@ struct SettingsView: View {
                     Text("Tapping a station either opens directions straight away or shows its details first.")
                 }
 
+                Section {
+                    Toggle("Enable Worth the Drive", isOn: fuelSavingsEnabledBinding)
+                    HStack {
+                        Text("Average fuel economy")
+                        Spacer()
+                        TextField("45", value: fuelSavingsMPGBinding, format: .number.precision(.fractionLength(0...1)))
+                            .keyboardType(.decimalPad)
+                            .multilineTextAlignment(.trailing)
+                            .frame(width: 80)
+                            .focused($focusedSavingsField, equals: .averageMPG)
+                        Text("MPG")
+                            .foregroundStyle(.secondary)
+                    }
+                    Picker("Fill-up unit", selection: fuelSavingsUnitBinding) {
+                        ForEach(FuelSavingsFillUnit.allCases) { unit in
+                            Text(unit.displayName).tag(unit)
+                        }
+                    }
+                    .pickerStyle(.segmented)
+                    HStack {
+                        Text("Typical fill-up")
+                        Spacer()
+                        TextField("40", value: fuelSavingsAmountBinding, format: .number.precision(.fractionLength(0...1)))
+                            .keyboardType(.decimalPad)
+                            .multilineTextAlignment(.trailing)
+                            .frame(width: 90)
+                            .focused($focusedSavingsField, equals: .fillAmount)
+                    }
+                    if !fuelSavingsSettings.isConfigured && fuelSavingsSettings.enabled {
+                        Text("Enter a realistic UK MPG and fill-up amount before Worth the Drive appears in sorting.")
+                            .font(.caption)
+                            .foregroundStyle(.orange)
+                    }
+                } header: {
+                    Text("Fuel Savings")
+                } footer: {
+                    Text("Worth the Drive estimates fuel cost only, using UK imperial MPG, your typical purchase amount, fuel prices and Apple Maps road distances. Real-world average MPG is better than manufacturer figures; the fill-up amount is your usual purchase, not tank capacity.")
+                }
+
                 Section {
                     Picker("Data checking", selection: $dataRefreshMode) {
                         ForEach(DataRefreshMode.allCases) { mode in
@@ -446,6 +496,9 @@ struct SettingsView: View {
                 }
 
             }
+            // Numeric entry must be escapable: `.decimalPad` has no Return key.
+            .keyboardDoneButton { focusedSavingsField = nil }
+            .scrollDismissesKeyboard(.interactively)
             .navigationTitle("Settings")
             .onAppear {
                 Task { await tipStore.load() }
@@ -565,6 +618,47 @@ struct SettingsView: View {
         return "\(version) (\(build))"
     }
 
+    private var fuelSavingsEnabledBinding: Binding<Bool> {
+        Binding(
+            get: { fuelSavingsSettings.enabled },
+            set: { fuelSavingsSettings.enabled = $0; persistFuelSavingsSettings() }
+        )
+    }
+
+    private var fuelSavingsMPGBinding: Binding<Double> {
+        Binding(
+            get: { fuelSavingsSettings.mpg },
+            set: { fuelSavingsSettings.mpg = min(max($0, 0), FuelSavingsCalculator.maximumMPG); persistFuelSavingsSettings() }
+        )
+    }
+
+    private var fuelSavingsAmountBinding: Binding<Double> {
+        Binding(
+            get: { fuelSavingsSettings.preferredAmount.value },
+            set: {
+                fuelSavingsSettings.preferredAmount.value = min(max($0, 0), FuelSavingsCalculator.maximumFillAmount(for: fuelSavingsSettings.preferredAmount.unit))
+                persistFuelSavingsSettings()
+            }
+        )
+    }
+
+    private var fuelSavingsUnitBinding: Binding<FuelSavingsFillUnit> {
+        Binding(
+            get: { fuelSavingsSettings.preferredAmount.unit },
+            set: { newUnit in
+                fuelSavingsSettings.preferredAmount.unit = newUnit
+                if !fuelSavingsSettings.preferredAmount.isValid {
+                    fuelSavingsSettings.preferredAmount.value = newUnit == .pounds ? 40 : 30
+                }
+                persistFuelSavingsSettings()
+            }
+        )
+    }
+
+    private func persistFuelSavingsSettings() {
+        FuelStore.saveFuelSavingsSettings(fuelSavingsSettings)
+    }
+
     /// "API" when the relay is serving the official Fuel Finder API, "CSV"
     /// when it fell back to the public mirror, "—" when unknown/never fetched.
     private var connectionText: String {