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/HowBestValueSheet.swift

@@ -16,6 +16,8 @@ struct HowBestValueSheet: View {
                     VStack(alignment: .leading, spacing: 10) {
                         Text("Best Value compares each station with the closest eligible station by journey distance. It works out what you could save at the pump, then takes off the fuel used for the extra return journey.")
                         Text("Your fill-up amount and MPG are used to show whether you could save overall.")
+                        Text("With a £ budget you spend that amount at each station; the litres differ by station. Savings compare those litres with the same quantity at your closest eligible station, then deduct extra journey fuel. You do not receive cash back.")
+                        Text("With Litres, every station is compared for the fixed quantity you selected.")
                         Text("Results are marked “Approx.” until road distances are available.")
                             .foregroundStyle(.secondary)
                     }