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/Bluetooth/BluetoothManager.swift

@@ -240,6 +240,10 @@ final class BluetoothManager: NSObject {
     // MARK: - Sonstiges
 
     private let store: DeviceStore
+    /// Von aussen gesetzt, sobald die App bereit ist. Optional, damit
+    /// `BluetoothManager` nichts über Live Activities wissen muss, wenn keine
+    /// läuft.
+    var activityManager: LevelActivityManager?
     private let isDemo = DemoData.isEnabled
     /// Wieviele Messpunkte je Gerät im Verlauf behalten werden.
     private let historyLimit = 720
@@ -892,7 +896,10 @@ extension BluetoothManager: CBCentralManagerDelegate {
                     self?.publish { self?.linkStates[device.id] = state }
                 },
                 onLevelState: { [weak self] state in
-                    self?.publish { self?.levelStates[device.id] = state }
+                    self?.publish {
+                        self?.levelStates[device.id] = state
+                        self?.activityManager?.update(deviceID: device.id, state: state)
+                    }
                 },
                 onDeviceOrientation: { [weak self] orientation in
                     // Im Gerät steht, wie der Sensor eingebaut ist – für alle