Fuel Savings: the feature's own settings grey out when it is switched off

With "Enable Worth the Drive" off, the MPG slider, the fill-up unit picker
and the fill-up slider stayed live — editable settings for a switched-off
feature. They now sit in a Group disabled by `enabled`, so they grey out but
stay on screen and keep their values (the same rule as the distance picker
in Closest mode), and re-enabling restores exactly what was there.

The surfaces that USE the feature were already gated on `isConfigured`
(which includes `enabled`), so the in-list fill-up control and the Worth the
Drive sort option still disappear entirely rather than greying out.

Verified on the simulator with the toggle off vs forced on: the MPG slider
row and the unit picker row both measurably dim — MPG row mean RGB
33.06/35.62/39.49 off vs 37.61/42.70/48.43 on; picker 40.00/40.00/42.62 vs
53.35/53.36/56.82 — while the toggle row itself changes as expected
(54.85/64.77/58.96 with the switch on).

FuelBoard/SettingsView.swift

@@ -151,41 +151,51 @@ struct SettingsView: View {
 
                 Section {
                     Toggle("Enable Worth the Drive", isOn: fuelSavingsEnabledBinding)
-                    HStack {
+                    // The feature's own controls follow its toggle: switched
-                        Text("Average fuel economy")
+                    // off they grey out but stay on screen (same rule as the
-                        Spacer()
+                    // distance picker in Closest mode) and keep their values, so
-                        Text(mpgText)
+                    // re-enabling restores exactly what was there. The sections
-                            .monospacedDigit()
+                    // that USE the feature — the in-list fill-up control and the
-                            .foregroundStyle(.secondary)
+                    // Worth the Drive sort option — are gated on `isConfigured`,
-                    }
+                    // which already includes `enabled`, so they hide entirely.
-                    Slider(value: fuelSavingsMPGBinding,
+                    Group {
-                           in: FuelSavingsCalculator.mpgSliderRange,
+                        HStack {
-                           step: FuelSavingsCalculator.mpgSliderStep)
+                            Text("Average fuel economy")
-                        .accessibilityLabel("Average fuel economy")
+                            Spacer()
-                        .accessibilityValue(mpgText)
+                            Text(mpgText)
-                    Picker("Fill-up unit", selection: fuelSavingsUnitBinding) {
+                                .monospacedDigit()
-                        ForEach(FuelSavingsFillUnit.allCases) { unit in
+                                .foregroundStyle(.secondary)
-                            Text(unit.displayName).tag(unit)
+                        }
+                        Slider(value: fuelSavingsMPGBinding,
+                               in: FuelSavingsCalculator.mpgSliderRange,
+                               step: FuelSavingsCalculator.mpgSliderStep)
+                            .accessibilityLabel("Average fuel economy")
+                            .accessibilityValue(mpgText)
+                        Picker("Fill-up unit", selection: fuelSavingsUnitBinding) {
+                            ForEach(FuelSavingsFillUnit.allCases) { unit in
+                                Text(unit.displayName).tag(unit)
+                            }
+                        }
+                        .pickerStyle(.segmented)
+                        HStack {
+                            Text("Typical fill-up")
+                            Spacer()
+                            Text(fillUpText)
+                                .monospacedDigit()
+                                .foregroundStyle(.secondary)
+                        }
+                        Slider(value: fuelSavingsAmountBinding,
+                               in: FuelSavingsCalculator.fillAmountSliderRange(for: fuelSavingsSettings.preferredAmount.unit),
+                               step: FuelSavingsCalculator.fillAmountSliderStep(for: fuelSavingsSettings.preferredAmount.unit))
+                            .accessibilityLabel("Typical fill-up")
+                            .accessibilityValue(fillUpText)
+                        if !fuelSavingsSettings.isConfigured && fuelSavingsSettings.enabled {
+                            Text("Set a realistic UK MPG and fill-up amount before Worth the Drive appears in sorting.")
+                                .font(.caption)
+                                .foregroundStyle(.orange)
                         }
                     }
-                    .pickerStyle(.segmented)
+                    .disabled(!fuelSavingsSettings.enabled)
-                    HStack {
-                        Text("Typical fill-up")
-                        Spacer()
-                        Text(fillUpText)
-                            .monospacedDigit()
-                            .foregroundStyle(.secondary)
-                    }
-                    Slider(value: fuelSavingsAmountBinding,
-                           in: FuelSavingsCalculator.fillAmountSliderRange(for: fuelSavingsSettings.preferredAmount.unit),
-                           step: FuelSavingsCalculator.fillAmountSliderStep(for: fuelSavingsSettings.preferredAmount.unit))
-                        .accessibilityLabel("Typical fill-up")
-                        .accessibilityValue(fillUpText)
-                    if !fuelSavingsSettings.isConfigured && fuelSavingsSettings.enabled {
-                        Text("Set a realistic UK MPG and fill-up amount before Worth the Drive appears in sorting.")
-                            .font(.caption)
-                            .foregroundStyle(.orange)
-                    }
                 } header: {
                     Text("Fuel Savings")
                 } footer: {