Add cheapest-favourite price-drop alerts

Shared/FuelStore.swift

@@ -31,6 +31,16 @@ struct FavouriteEntry: Identifiable, Codable, Equatable {
     var id: String { "\(fuel.rawValue)|\(station.id)" }
 }
 
+/// Persisted baseline for the per-fuel cheapest-favourite price-drop alert.
+/// Stored separately from the favourites list so refresh-time comparison can
+/// survive app relaunches without immediately alerting on first fetch.
+struct FavouriteAlertSnapshot: Codable, Equatable {
+    let fuel: FuelType
+    let stationID: String
+    let stationName: String
+    let price: Double
+}
+
 enum SortMode: String, Codable, CaseIterable, Identifiable {
     case cheapest
     case closest
@@ -318,6 +328,9 @@ struct FuelStore {
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
     static let alertsFuelKey = "fuelboard.alertsFuel"       // FuelType raw value
     static let alertsFollowSearchKey = "fuelboard.alertsFollowSearch"  // Bool — alerts mirror the Stations-tab distance
+    static let favouriteDropAlertsEnabledKey = "fuelboard.favouriteDropAlertsEnabled" // Bool
+    static let favouriteDropAlertsFuelKey = "fuelboard.favouriteDropAlertsFuel" // FuelType raw value
+    static let favouriteAlertSnapshotsKey = "fuelboard.favouriteAlertSnapshots" // [fuel: FavouriteAlertSnapshot] JSON
     static let liveActivityFollowSearchKey = "fuelboard.liveActivityFollowSearch" // Bool — Live Activity mirrors the Stations-tab distance
     static let debugModeKey = "fuelboard.debugMode"         // Bool — hidden dev flag
     static let relayFallbackKey = "fuelboard.relayFallback" // Bool — dev-only relay fallback
@@ -627,6 +640,24 @@ struct FuelStore {
         }
     }
 
+    /// The cheapest favourite for one fuel, preserving the user's stored order
+    /// on price ties. Used by the favourite price-drop alert so a tie does not
+    /// flap between equally-priced favourites.
+    static func cheapestFavourite(in favourites: [FavouriteEntry], fuel: FuelType) -> FavouriteEntry? {
+        var best: FavouriteEntry?
+        for entry in favourites where entry.fuel == fuel {
+            guard let price = entry.station.prices[fuel] else { continue }
+            guard let current = best, let currentPrice = current.station.prices[fuel] else {
+                best = entry
+                continue
+            }
+            if price < currentPrice {
+                best = entry
+            }
+        }
+        return best
+    }
+
     /// 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
@@ -710,6 +741,48 @@ struct FuelStore {
         saveString(enabled ? "1" : "0", service: alertsFollowSearchKey)
     }
 
+    // MARK: Favourite price-drop alerts
+
+    static func loadFavouriteDropAlertsEnabled() -> Bool {
+        loadString(service: favouriteDropAlertsEnabledKey) == "1"
+    }
+
+    static func saveFavouriteDropAlertsEnabled(_ enabled: Bool) {
+        saveString(enabled ? "1" : "0", service: favouriteDropAlertsEnabledKey)
+    }
+
+    static func loadFavouriteDropAlertsFuel() -> FuelType {
+        if let raw = loadString(service: favouriteDropAlertsFuelKey), let fuel = FuelType(rawValue: raw) {
+            return fuel
+        }
+        return .e10
+    }
+
+    static func saveFavouriteDropAlertsFuel(_ fuel: FuelType) {
+        saveString(fuel.rawValue, service: favouriteDropAlertsFuelKey)
+    }
+
+    static func loadFavouriteAlertSnapshots() -> [FuelType: FavouriteAlertSnapshot] {
+        if let data = keychainData(service: favouriteAlertSnapshotsKey),
+           let stored = try? JSONDecoder().decode([String: FavouriteAlertSnapshot].self, from: data) {
+            var result: [FuelType: FavouriteAlertSnapshot] = [:]
+            for (key, value) in stored {
+                if let fuel = FuelType(rawValue: key) {
+                    result[fuel] = value
+                }
+            }
+            return result
+        }
+        return [:]
+    }
+
+    static func saveFavouriteAlertSnapshots(_ snapshots: [FuelType: FavouriteAlertSnapshot]) {
+        let keyed = Dictionary(uniqueKeysWithValues: snapshots.map { ($0.key.rawValue, $0.value) })
+        if let data = try? JSONEncoder().encode(keyed) {
+            writeKeychain(data: data, service: favouriteAlertSnapshotsKey)
+        }
+    }
+
     // MARK: Live Activity
 
     static func loadLiveActivityEnabled() -> Bool {