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/LiveActivityManager.swift

@@ -57,7 +57,7 @@ enum LiveActivityManager {
             brand: best.brand,
             pricePence: price,
             priceDisplayStyle: priceDisplayStyle ?? FuelStore.loadPriceDisplayStyle(),
-            distanceKM: best.distanceKM(to: location.lat, lng2: location.lng),
+            distanceKM: FuelStore.displayDistanceKM(station: best, userLat: location.lat, userLng: location.lng),
             lat: best.lat,
             lng: best.lng,
             updatedAt: Date()