Add Live Activity for level sensor (surfaces on CarPlay automatically)

Replaces the ActivityKit boilerplate with a real lock screen/Dynamic
Island UI driven by live pitch/roll readings, toggled from the
leveling device's detail screen. Since iOS 17 shows any running Live
Activity on CarPlay's dashboard without a dedicated CarPlay app entitlement,
this covers the requested CarPlay use case without one.

Also fixes the widget extension never being embedded into the app
(missing Embed Foundation Extensions build phase and target
dependency), which meant no widget or Live Activity could ever have
rendered regardless of code changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CamperMonitor/CamperMonitorApp.swift

@@ -5,14 +5,18 @@ struct CamperMonitorApp: App {
     @State private var store: DeviceStore
     @State private var bluetooth: BluetoothManager
     @State private var watch: PhoneWatchLink
+    @State private var levelActivity: LevelActivityManager
     @Environment(\.scenePhase) private var scenePhase
 
     init() {
         let store = DeviceStore()
         let bluetooth = BluetoothManager(store: store)
+        let levelActivity = LevelActivityManager()
+        bluetooth.activityManager = levelActivity
         _store = State(initialValue: store)
         _bluetooth = State(initialValue: bluetooth)
         _watch = State(initialValue: PhoneWatchLink(store: store, bluetooth: bluetooth))
+        _levelActivity = State(initialValue: levelActivity)
     }
 
     var body: some Scene {
@@ -21,6 +25,7 @@ struct CamperMonitorApp: App {
                 .environment(store)
                 .environment(bluetooth)
                 .environment(watch)
+                .environment(levelActivity)
                 .task { watch.activate() }
         }
         .onChange(of: scenePhase) { _, phase in