widget + live activity: prefer Apple-Maps road distance, computed by the app

The widget/Live Activity showed straight-line haversine distance (0.8 mi)
while Apple Maps routes 1.8 mi. Road routing is too heavy for the widget
execution + ~40-70/day refresh budget, so the APP now computes it:

- New RoadDistanceService (app target): for the nearest 12 stations each
  pass, calls MapKit MKDirections (free, no API key, matches Apple Maps)
  and caches metres keyed by station ID.
- Cache stored in KEYCHAIN (fuelboard.roadDistances) so the widget reads it
  even on free SideStore accounts with no app-group container; only valid
  within 600 m of the location it was built from.
- Throttled: recompute max every 10 min, or when the user moves > 400 m;
  wired into the location-update hook + location onChange.
- Widget face, app station rows + Live Activity show the cached road
  distance, falling back to straight-line when absent.
- 4 new cache tests (105 total).

FuelBoard/ContentView.swift

@@ -50,6 +50,21 @@ struct ContentView: View {
             // fires on every fix incl. background significant-change
             // wake-ups, so the Lock Screen pill stays live while driving.
             updateLiveActivity()
+            refreshRoadDistancesIfNeeded()
+        }
+    }
+
+    /// Kicks off a (throttled) Apple-Maps road-distance recompute for the
+    /// stations around the current fix. The app owns routing — the widget and
+    /// Live Activity only read the cached result.
+    private func refreshRoadDistancesIfNeeded() {
+        guard let location else { return }
+        Task {
+            await RoadDistanceService.refreshIfNeeded(
+                stations: stations,
+                lat: location.lat,
+                lng: location.lng
+            )
         }
     }
 
@@ -377,6 +392,7 @@ struct ContentView: View {
                 monitor.update(stations: stations, favourites: refreshedFavourites,
                                fuel: alertsFuel, radiusKM: effectiveAlertsRadiusKM)
                 updateLiveActivity()
+                refreshRoadDistancesIfNeeded()
             }
         }
         )
@@ -949,7 +965,7 @@ struct StationRow: View {
                     .lineLimit(1)
                     .truncationMode(.tail)
                 if let location {
-                    Text(distanceUnit.format(station.distanceKM(to: location.lat, lng2: location.lng)))
+                    Text(distanceUnit.format(FuelStore.displayDistanceKM(station: station, userLat: location.lat, userLng: location.lng)))
                         .font(.caption2)
                         .foregroundStyle(.secondary)
                         .monospacedDigit()