Best Value: explain price age per surface instead of one blanket warning

- Station detail now separates the station's own report age from the
  selected fuel's update time, so a recent station-wide report no longer
  reads as an old price.
- Warning copy names the fuel and says which side is weak: the station's
  price is not individually dated, or the baseline's is. The misleading
  "unverified or older than 7 days" line is gone.
- Wording stays "Closest" throughout; new copy uses Unleaded / Premium /
  Diesel rather than the internal grade codes.
- Localisation: new and updated keys for the added copy.

FuelBoard/SettingsView.swift

@@ -79,6 +79,7 @@ struct SettingsView: View {
     /// Return key, so the keyboard's Done button clears this to dismiss AND
     /// commit the typed value. Both Fuel Savings controls are sliders now, so
     /// nothing sets it — it stays for any numeric field this screen takes again.
+    @State private var settingsUnitChangeNotice: String?
     @FocusState private var focusedSavingsField: SavingsField?
 
     private enum SavingsField: Hashable {
@@ -148,6 +149,11 @@ struct SettingsView: View {
                                step: FuelSavingsCalculator.fillAmountSliderStep(for: fuelSavingsSettings.preferredAmount.unit))
                             .accessibilityLabel("Typical fill-up")
                             .accessibilityValue(fillUpText)
+                        if let settingsUnitChangeNotice {
+                            Text(settingsUnitChangeNotice)
+                                .font(.caption2)
+                                .foregroundStyle(.orange)
+                        }
                         if !fuelSavingsSettings.isConfigured && fuelSavingsSettings.enabled {
                             Text("Set a realistic UK MPG and fill-up amount before Best Value appears in sorting.")
                                 .font(.caption)
@@ -709,10 +715,10 @@ struct SettingsView: View {
         Binding(
             get: { fuelSavingsSettings.preferredAmount.unit },
             set: { newUnit in
-                fuelSavingsSettings.preferredAmount.unit = newUnit
-                if !fuelSavingsSettings.preferredAmount.isValid {
-                    fuelSavingsSettings.preferredAmount.value = newUnit == .pounds ? 40 : 30
-                }
+                guard newUnit != fuelSavingsSettings.preferredAmount.unit else { return }
+                fuelSavingsSettings.preferredAmount = FuelSavingsCalculator.newComparisonAmount(
+                    afterSwitching: fuelSavingsSettings.preferredAmount, to: newUnit)
+                settingsUnitChangeNotice = "New comparison basis — adjust the \(newUnit.displayName) amount to your intended fill-up."
                 persistFuelSavingsSettings()
             }
         )