Tied cheapest stations become notification action buttons

When several stations share the cheapest price within the radius (within
0.01p), the alert registers a dynamic UNNotificationCategory with up to
4 action buttons (brand + distance, nearest first) and taps open
directions to the chosen station. Shared FuelStore.tiedStations helper
used by both the live geofence path and the Settings real-data test;
35 tests pass.

Shared/FuelStore.swift

@@ -257,6 +257,22 @@ struct FuelStore {
     // defaults-first for stations (keychain may hold a legacy small set from
     // older builds; the full country dump always wins).
 
+    /// Stations from `stations` tied with `price` for `fuel` (within 0.01p),
+    /// sorted nearest-first from `lat`/`lng`. Used to offer multiple
+    /// cheapest-station choices in an alert notification.
+    static func tiedStations(
+        in stations: [FuelStation],
+        fuel: FuelType,
+        price: Double,
+        fromLat lat: Double, lng: Double
+    ) -> [FuelStation] {
+        stations
+            .filter { abs(($0.prices[fuel] ?? .infinity) - price) < 0.01 }
+            .sorted { lhs, rhs in
+                lhs.distanceKM(to: lat, lng2: lng) < rhs.distanceKM(to: lat, lng2: lng)
+            }
+    }
+
     static func loadStations() -> [FuelStation] {
         if let defaults = UserDefaults(suiteName: appGroupSuite),
            let data = defaults.data(forKey: stationsKey),