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/KeyboardSupport.swift

@@ -1,5 +1,6 @@
 import SwiftUI
 import UIKit
+import Combine
 
 extension View {
     /// Puts a Done button above the keyboard.
@@ -11,31 +12,68 @@ extension View {
     /// `TextField(value:format:)` only commits the typed value when editing
     /// ends — so the number never stuck either.
     ///
-    /// Apply ONCE per screen, to the list or form containing the fields: the
-    /// keyboard toolbar belongs to the focused field, so a single attachment
-    /// covers every field inside it. Attaching it to each field would show
-    /// several Done buttons at once.
+    /// WHY IT IS DRAWN HERE RATHER THAN AS `.toolbar(placement: .keyboard)`:
+    /// the keyboard-toolbar form is device-dependent. The same build showed the
+    /// Done button on the iPhone 15 Pro and NOT on the iPhone SE (both iOS
+    /// 27.0.1, same install, same binary), leaving the SE with no way to
+    /// dismiss or commit a numeric field. Drawing the bar in the screen's own
+    /// bottom safe area renders identically on every device, so the accessibility
+    /// of the numeric fields no longer depends on the accessory view appearing.
+    ///
+    /// Apply ONCE per screen, to the list or form containing the fields: any
+    /// keyboard on that screen shows the bar, so attaching it to each field
+    /// would stack several Done buttons.
     ///
     /// - Parameter action: clears the screen's focus state. Prefer this where a
     ///   `@FocusState` exists; the default resigns the current first responder,
-    ///   which works for fields inside subviews that have no focus binding.
+    ///   which works for fields inside subviews that have no focus binding (the
+    ///   fill-up box in the station list is one).
     func keyboardDoneButton(action: (() -> Void)? = nil) -> some View {
-        toolbar {
-            ToolbarItemGroup(placement: .keyboard) {
-                Spacer()
-                Button("Done") {
-                    if let action {
-                        action()
-                    } else {
-                        UIApplication.shared.sendAction(
-                            #selector(UIResponder.resignFirstResponder),
-                            to: nil,
-                            from: nil,
-                            for: nil
-                        )
-                    }
-                }
-            }
-        }
+        modifier(KeyboardDoneBar(action: action))
+    }
+}
+
+/// Shows a Done bar while a keyboard is up on this screen.
+///
+/// Visibility comes from the keyboard notifications rather than a `@FocusState`
+/// so the bar also works for fields inside subviews that have no focus binding.
+private struct KeyboardDoneBar: ViewModifier {
+    let action: (() -> Void)?
+
+    @State private var isKeyboardVisible = false
+
+    func body(content: Content) -> some View {
+        content
+            .safeAreaInset(edge: .bottom, spacing: 0) {
+                if isKeyboardVisible {
+                    HStack {
+                        Spacer()
+                        Button("Done") {
+                            if let action {
+                                action()
+                            } else {
+                                UIApplication.shared.sendAction(
+                                    #selector(UIResponder.resignFirstResponder),
+                                    to: nil,
+                                    from: nil,
+                                    for: nil
+                                )
+                            }
+                            isKeyboardVisible = false
+                        }
+                        .font(.body.weight(.semibold))
+                    }
+                    .padding(.horizontal, 20)
+                    .padding(.vertical, 10)
+                    .frame(maxWidth: .infinity)
+                    .background(.bar)
+                }
+            }
+            .onReceive(NotificationCenter.default.publisher(for: UIResponder.keyboardWillShowNotification)) { _ in
+                isKeyboardVisible = true
+            }
+            .onReceive(NotificationCenter.default.publisher(for: UIResponder.keyboardWillHideNotification)) { _ in
+                isKeyboardVisible = false
+            }
     }
 }