Worth the Drive: calculation audit + listing/detail polish

Calculation audit (brief config: 30 MPG imperial, £60 fill-up, baseline
Rontec Shelf 171.9p at 0.4 mi):

- One mile<->kilometre constant pair (kilometresPerMile 1.609344,
  metresPerMile 1609.344) replaces the mixed 1609.34 / 0.621371 / 1.60934
  used in the savings path. Effect ~2 ppm (invisible), but the conversions
  are now auditable in one place.
- New FuelSavingsCalculationAuditTests (20 deterministic tests, no live
  MKDirections) pins the brief's three stations - Tesco 170.9p/2.0 mi,
  MFG Hare & Hounds 172.9p/2.8 mi, PGG Napoleon 171.9p/2.8 mi - plus
  imperial-vs-US gallon, MPG and budget scaling, the break-even tolerance
  boundary, the nearer-candidate clamp, route-unavailable, mixed-basis
  estimates and display-rounding boundaries.
- Formula unchanged, and verified correct: fixed quantity (budget / baseline
  price), pump saving, round-trip extra distance clamped at zero, travel fuel
  priced at the candidate station, +/-5p break even, baseline labelled
  "Closest station", sorting on the unrounded net saving. The brief's "£0.18"
  for Tesco is the rounded 1 dp extra-distance caption (2.02-2.05 mi reads as
  "2.0 mi"), not a rounding bug in the money.

UI (uncommitted since d7a0ecb):

- Done bar drawn in the bottom safe area from keyboard notifications:
  .toolbar(placement: .keyboard) is device-dependent (present on the 15 Pro,
  absent on the SE on the same iOS build).
- Favourite star in the More-info heading, wired to the same fuel-scoped
  toggle as the listing rows at all three call sites.
- CHEAPEST/TOP badge moved above the price so it reads as part of it.
- Listing rows carry the real session fill-up amount instead of a hard zero
  (the compact banner does not print it today; a future row layout would).

153 tests, 0 failures. Simulator and device builds succeed.

FuelBoard/StationDetailView.swift

@@ -13,6 +13,12 @@ struct StationDetailView: View {
     var fuelSavings: FuelSavingsAvailability? = nil
     var fuelSavingsAmount: FuelSavingsFillAmount = FuelSavingsSettings.defaultPreferredAmount
     var savingsBaselineStationName: String? = nil
+    /// Whether this station is starred for the fuel currently being viewed, and
+    /// the toggle that flips it. Passed from the list that opened the sheet, so
+    /// the star here and the star on the row are the same fuel-scoped
+    /// favourite and cannot drift apart.
+    var isFavourite: Bool = false
+    var onToggleFavourite: (() -> Void)? = nil
     @Environment(\.dismiss) private var dismiss
     @State private var showingReportIssueConfirmation = false
 
@@ -61,7 +67,8 @@ struct StationDetailView: View {
                     FuelSavingsSummaryView(
                         availability: fuelSavings,
                         amount: fuelSavingsAmount,
-                        baselineStationName: savingsBaselineStationName
+                        baselineStationName: savingsBaselineStationName,
+                        distanceUnit: distanceUnit
                     )
                 } header: {
                     Text("Worth the drive?")
@@ -96,6 +103,20 @@ struct StationDetailView: View {
                 ToolbarItem(placement: .cancellationAction) {
                     Button("Done") { dismiss() }
                 }
+                ToolbarItem(placement: .confirmationAction) {
+                    Button {
+                        onToggleFavourite?()
+                    } label: {
+                        Image(systemName: isFavourite ? "star.fill" : "star")
+                    }
+                    .tint(isFavourite ? Color.yellow : nil)
+                    .disabled(onToggleFavourite == nil)
+                    .accessibilityLabel(
+                        isFavourite
+                            ? String(localized: "Remove from favourites", comment: "Unstar a station from the More info sheet")
+                            : String(localized: "Add to favourites", comment: "Star a station from the More info sheet")
+                    )
+                }
             }
         }
     }