Add Worth the Drive fuel savings with road-routed estimates

Optional Fuel Savings feature (off by default): vehicle MPG in UK imperial gallons, fill amount in pounds or litres, persisted through the existing keychain/app-group store. Ranks the WHOLE station pool by estimated net saving (fuel-price saving at each pump minus the extra return-trip driving cost) using MapKit road distances with a straight-line fallback marked (est.).

The closest station with a valid price for the selected fuel is the comparison baseline and is labelled "Closest station"; other rows read "Save £X.XX after travel" / "£X.XX worse off after travel" / "Break even". Labels live in the shared model so they are unit-tested. The TOP badge still marks the cheapest pump price and reads "CHEAPEST" when the list is ranked by net saving.

Road routing now prioritises the Worth-the-Drive candidates plus the baseline and merges cache entries instead of replacing them, so far-out stations get real routes rather than permanent straight-line estimates. Numeric fields gain a Done button above the keypad (decimalPad has no Return key); the fill-up control is hidden unless the feature is configured; Favourites shares one session fill amount with the station list. 131 tests.

FuelBoard/KeyboardSupport.swift

@@ -0,0 +1,41 @@
+import SwiftUI
+import UIKit
+
+extension View {
+    /// Puts a Done button above the keyboard.
+    ///
+    /// The numeric fields (Settings → Average fuel economy and Typical
+    /// fill-up, and the Worth the Drive fill-up box in the station list) use
+    /// `.decimalPad`, which has NO Return key. Without this there is no way to
+    /// dismiss the keyboard once such a field is tapped, and a
+    /// `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.
+    ///
+    /// - 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.
+    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
+                        )
+                    }
+                }
+            }
+        }
+    }
+}