Widget Favourites filter: pinned stations cheapest-first, no radius

- New Sort by value 'Favourites' in Edit Widget (per-widget config).
- ParameterSummary hides the Distance picker when Favourites is selected
  (favourites are not radius-bound).
- Favourites mode reads keychain favourites (shared access group works on
  SideStore free even without App Groups), filters to the widget's fuel,
  refreshes prices from a focused relay fetch when in range, and ranks
  cheapest-first with distance tiebreak. Not radius-filtered.
- App now re-saves refreshed favourites to keychain after every fetch so
  the widget shows current prices instead of star-time snapshots.
- Empty state + headings adapt to favourites mode. 35 tests pass.

FuelBoard/ContentView.swift

@@ -285,6 +285,11 @@ struct ContentView: View {
             stations = fetched
             FuelStore.saveStations(fetched)
             FuelStore.saveLastRefresh()
+            // Keep the keychain favourites fresh with the new prices — the
+            // widget's Favourites mode reads them from keychain (the only
+            // channel shared on SideStore free), so stale star-time snapshots
+            // would otherwise show old prices.
+            FuelStore.saveFavourites(refreshedFavourites)
             WidgetCenter.shared.reloadAllTimelines()
             statusMessage = "Loaded \(fetched.count) stations · \(Date().formatted(date: .omitted, time: .shortened))"
         } catch {