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/FuelSavingsViews.swift

@@ -2,7 +2,8 @@ import SwiftUI
 
 struct FuelSavingsAdjustmentView: View {
     @Binding var amount: FuelSavingsFillAmount
-    let referencePricePencePerLitre: Double?
+
+    @State private var unitChangeNotice: String?
 
     private var range: ClosedRange<Double> {
         FuelSavingsCalculator.fillAmountSliderRange(for: amount.unit)
@@ -44,6 +45,14 @@ struct FuelSavingsAdjustmentView: View {
             Text("Changes the estimate for this session only — your Settings default stays unchanged.")
                 .font(.caption2)
                 .foregroundStyle(.secondary)
+            Text("£ compares the litres your budget buys at each station; Litres compares one fixed quantity.")
+                .font(.caption2)
+                .foregroundStyle(.secondary)
+            if let unitChangeNotice {
+                Text(unitChangeNotice)
+                    .font(.caption2)
+                    .foregroundStyle(.orange)
+            }
         }
     }
 
@@ -52,12 +61,10 @@ struct FuelSavingsAdjustmentView: View {
             get: { amount.unit },
             set: { newUnit in
                 guard newUnit != amount.unit else { return }
-                if let referencePricePencePerLitre,
-                   let converted = try? FuelSavingsCalculator.convert(amount: amount, to: newUnit, referencePricePencePerLitre: referencePricePencePerLitre) {
-                    amount = converted
-                } else {
-                    amount = FuelSavingsFillAmount(value: newUnit == .pounds ? 40 : 30, unit: newUnit)
-                }
+                // No one litre figure equals a budget at every candidate price.
+                // A unit change starts a new, explicitly labelled comparison.
+                amount = FuelSavingsCalculator.newComparisonAmount(afterSwitching: amount, to: newUnit)
+                unitChangeNotice = "New comparison basis — adjust the \(newUnit.displayName) amount to your intended fill-up."
             }
         )
     }
@@ -73,6 +80,9 @@ struct FuelSavingsAdjustmentView: View {
 struct FuelSavingsSummaryView: View {
     let availability: FuelSavingsAvailability?
     let amount: FuelSavingsFillAmount
+    /// Candidate/baseline price confidence for every Best Value result.
+    var priceConfidence: FuelPriceFreshness.ComparisonConfidence = .ineligible
+    var priceWarning: String? = nil
     var baselineStationName: String? = nil
     var compact: Bool = false
     /// Sizes the "extra return travel" caption in the user's own unit (the
@@ -107,6 +117,12 @@ struct FuelSavingsSummaryView: View {
                             .layoutPriority(1)
                             .accessibilityLabel("Comparison: \(comparisonLabel(for: result))")
                     }
+                    if let warning = priceWarning ?? priceConfidence.warningText {
+                        Text(warning)
+                            .font(.caption2)
+                            .foregroundStyle(.orange)
+                            .accessibilityLabel(warning)
+                    }
                     if let extraTravel = FuelSavingsFormat.extraTravelLabel(
                         extraDistanceMiles: result.extraDistanceMiles,
                         unit: distanceUnit,
@@ -143,6 +159,17 @@ struct FuelSavingsSummaryView: View {
                     LabeledContent("Your fill-up", value: amountText)
                     LabeledContent("Fuel you'll receive", value: String(format: "%.2f litres", result.fillLitres))
                         .monospacedDigit()
+                    if amount.unit == .pounds {
+                        Text("You pay the selected budget here. The saving compares the same litres at your closest eligible station; litres differ between stations, and this is not a refund.")
+                            .font(.caption2)
+                            .foregroundStyle(.secondary)
+                    }
+                    if let warning = priceWarning ?? priceConfidence.warningText {
+                        Text(warning)
+                            .font(.caption2)
+                            .foregroundStyle(.orange)
+                            .accessibilityLabel(warning)
+                    }
                     if let extraTravel = FuelSavingsFormat.extraTravelLabel(
                         extraDistanceMiles: result.extraDistanceMiles,
                         unit: distanceUnit,
@@ -158,7 +185,7 @@ struct FuelSavingsSummaryView: View {
                             .font(.caption)
                             .foregroundStyle(.secondary)
                     } else {
-                        Text("Compared with \(baselineStationName ?? "your closest station").")
+                        Text(baselineStationName.map { "Compared with \($0)." } ?? "Compared with your closest eligible station.")
                             .font(.caption)
                             .foregroundStyle(.secondary)
                         LabeledContent(
@@ -233,7 +260,7 @@ struct FuelSavingsSummaryView: View {
     }
 
     /// VoiceOver form of the verdict. `spokenHeadline()` spells "Approx." out for
-    /// estimated comparisons and leaves the plain words ("Break even", "Closest
+    /// and leaves the plain words ("Break even", "Closest eligible
     /// station") alone — one source, so visible and spoken copy cannot drift.
     private func accessibilityHeadline(for result: FuelSavingsResult) -> String {
         result.spokenHeadline()