Harden watch sync and App Group storage

FuelBoardWatch Watch App/ContentView.swift

@@ -157,6 +157,7 @@ private enum WatchFuelCache {
     static let appGroupSuite = "group.com.apt.fuelboard"
     static let favouritesKey = "fuelboard.favourites"
     static let stationsKey = "fuelboard.stations"
+    static let stationsFileName = "fuelboard.stations.json"
     static let fuelKey = "fuelboard.selectedFuel"
     static let distanceUnitKey = "fuelboard.distanceUnit"
     static let priceStyleKey = "fuelboard.priceDisplayStyle"
@@ -228,7 +229,17 @@ private enum WatchFuelCache {
     }
 
     static func loadStations() -> [WatchStation] {
-        guard let data = defaults()?.data(forKey: stationsKey),
+        let fileData: Data?
+        if let container = FileManager.default.containerURL(
+            forSecurityApplicationGroupIdentifier: appGroupSuite
+        ) {
+            let url = container.appendingPathComponent(stationsFileName)
+            fileData = try? Data(contentsOf: url)
+        } else {
+            fileData = nil
+        }
+        let data = fileData ?? defaults()?.data(forKey: stationsKey)
+        guard let data,
               let stations = try? JSONDecoder().decode([WatchStation].self, from: data) else {
             return []
         }