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).

Shared/FuelPriceWidgetViews.swift

@@ -89,7 +89,7 @@ struct FuelPriceWidgetContent: View {
                 FuelStore.priceTextAttributed(price, size: 26, weight: .bold, color: .green)
             }
             if let location = entry.location {
-                Text(entry.unit.format(station.distanceKM(to: location.lat, lng2: location.lng)) + " away")
+                Text(entry.unit.format(FuelStore.displayDistanceKM(station: station, userLat: location.lat, userLng: location.lng)) + " away")
                     .font(.caption2)
                     .foregroundStyle(.secondary)
             } else {
@@ -148,7 +148,7 @@ struct FuelPriceWidgetContent: View {
                 .font(.caption.weight(.semibold))
                 .lineLimit(1)
             if let location = entry.location {
-                Text(entry.unit.format(station.distanceKM(to: location.lat, lng2: location.lng)))
+                Text(entry.unit.format(FuelStore.displayDistanceKM(station: station, userLat: location.lat, userLng: location.lng)))
                     .font(.caption2)
                     .foregroundStyle(.secondary)
             }