Favourites: drag-and-drop reorder per fuel; stored order drives single-widget favourite

Shared/FuelStore.swift

@@ -490,6 +490,37 @@ struct FuelStore {
         }
     }
 
+    /// Reorders ONE fuel's favourites within the global array (drag-and-drop in
+    /// the Favourites tab). The moved fuel's block stays at its original
+    /// position in the array; other fuels keep their relative order. The array
+    /// order IS the widget order — the first favourite of a fuel is the
+    /// "single widget" favourite.
+    static func reorderedFavourites(_ favourites: [FavouriteEntry],
+                                    fuel: FuelType,
+                                    fromOffsets source: IndexSet,
+                                    toOffset destination: Int) -> [FavouriteEntry] {
+        var fuelEntries = favourites.filter { $0.fuel == fuel }
+        // Manual reorder (Foundation-only file — Array.move(fromOffsets:) is
+        // a SwiftUI helper). Reproduces the standard drag semantics: remove
+        // the source items, then insert at the destination, shifted by the
+        // number of removed items that were before it.
+        let moving = source.sorted()
+        let removed = moving.map { fuelEntries[$0] }
+        for index in moving.reversed() {
+            fuelEntries.remove(at: index)
+        }
+        var insertion = destination
+        for index in moving where index < destination {
+            insertion -= 1
+        }
+        fuelEntries.insert(contentsOf: removed, at: min(max(insertion, 0), fuelEntries.count))
+        let movedIDs = Set(fuelEntries.map(\.id))
+        var others = favourites.filter { !movedIDs.contains($0.id) }
+        let blockIndex = favourites.firstIndex { $0.fuel == fuel } ?? others.count
+        others.insert(contentsOf: fuelEntries, at: min(blockIndex, others.count))
+        return others
+    }
+
     // MARK: Alerts
 
     static func loadAlertsEnabled() -> Bool {