Live Activity: cheapest-in-radius Lock Screen/Dynamic Island pill

- Shared FuelBoardLiveActivityAttributes/ContentState (app + widget targets)
- FuelBoardLiveActivity Widget: ActivityConfiguration rendered by the widget
  extension - Lock Screen banner + Dynamic Island (leading/trailing/bottom,
  compact, minimal), tap anywhere -> maps:// directions to the best station
- LiveActivityManager (app side): request/update/end; mirrors the app list's
  TOP badge (selected fuel + chosen distance radius, price-then-distance
  ties); local start-date tracking for the 8h cap (this SDK's Activity has no
  startDate) - auto-restarts at 7h59m; silent no-op when disabled/denied
- Wiring: 250m/60s location hook (incl. background significant-change
  wake-ups), refresh completion, fuel/radius/unit changes, toggle flip
- Settings toggle + FuelStore persistence (keychain-backed, app-group)
- App-Info.plist: NSSupportsLiveActivities

Shared/FuelBoardLiveActivity.swift

@@ -0,0 +1,32 @@
+// FuelBoardLiveActivity.swift — Live Activity shared model.
+//
+// Compiled into BOTH the app target (to start/update/end the activity) and
+// the widget extension (to render it). ActivityKit is iOS-only, so this file
+// is deliberately NOT symlinked into FuelBoardTests/Sources — the SPM test
+// target runs on macOS where ActivityKit does not exist.
+//
+// The activity mirrors the app's TOP badge: the cheapest station selling the
+// app's selected fuel within the app's chosen distance radius. Tapping the
+// activity opens Apple Maps driving directions to that station (maps://).
+
+import ActivityKit
+import Foundation
+
+/// Attributes are fixed at request time. Here they pin the FUEL being
+/// tracked; everything that can change while driving (station, price,
+/// distance) lives in ContentState so updates don't need a new activity.
+struct FuelBoardLiveActivityAttributes: ActivityAttributes {
+    var fuel: FuelType
+
+    public struct ContentState: Codable, Hashable {
+        var stationID: String
+        var stationName: String
+        var brand: String
+        /// Price in pence per litre (same unit as `FuelStation.prices`).
+        var pricePence: Double
+        var distanceKM: Double
+        var lat: Double
+        var lng: Double
+        var updatedAt: Date
+    }
+}

Shared/FuelStore.swift

@@ -262,6 +262,7 @@ struct FuelStore {
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
     static let alertsFuelKey = "fuelboard.alertsFuel"       // FuelType raw value
     static let debugModeKey = "fuelboard.debugMode"         // Bool — hidden dev flag
+    static let liveActivityKey = "fuelboard.liveActivity"    // Bool — Live Activity toggle
     static let onboardingCompletedKey = "fuelboard.onboardingCompleted" // Bool
     static let lastRefreshKey = "fuelboard.lastRefresh"      // TimeInterval (seconds since 1970)
     static let relaySourceKey = "fuelboard.relaySource"      // String — "api" | "csv"
@@ -490,6 +491,16 @@ struct FuelStore {
         saveString(String(radius), service: alertsRadiusKey)
     }
 
+    // MARK: Live Activity
+
+    static func loadLiveActivityEnabled() -> Bool {
+        loadString(service: liveActivityKey) == "1"
+    }
+
+    static func saveLiveActivityEnabled(_ enabled: Bool) {
+        saveString(enabled ? "1" : "0", service: liveActivityKey)
+    }
+
     // MARK: Refresh policy — data is cached; the app only auto-refreshes
     // twice a day (pull-to-refresh is the manual override).