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.

FuelBoardWidgets/WidgetConfigIntent.swift

@@ -24,12 +24,14 @@ enum WidgetFuel: String, AppEnum, CaseIterable, Codable {
 enum WidgetSort: String, AppEnum, CaseIterable, Codable {
     case cheapest
     case closest
+    case favourites
 
     static var typeDisplayRepresentation: TypeDisplayRepresentation = "Sort by"
 
     static var caseDisplayRepresentations: [WidgetSort: DisplayRepresentation] = [
         .cheapest: "Cheapest",
         .closest: "Closest",
+        .favourites: "Favourites",
     ]
 }
 
@@ -67,4 +69,21 @@ struct FuelBoardWidgetConfigurationIntent: WidgetConfigurationIntent {
 
     @Parameter(title: "Distance", default: .five)
     var distance: WidgetDistance
+
+    /// Edit-Widget UI: Favourites ranks pinned stations cheapest-first and is
+    /// not radius-bound, so the Distance picker is hidden in that mode.
+    static var parameterSummary: some ParameterSummary {
+        When(\.$sort, .equalTo, WidgetSort.favourites) {
+            Summary("Show \(\.$fuel) favourites") {
+                \.$fuel
+                \.$sort
+            }
+        } otherwise: {
+            Summary("Show \(\.$fuel) by \(\.$sort) within \(\.$distance)") {
+                \.$fuel
+                \.$sort
+                \.$distance
+            }
+        }
+    }
 }