Add in-app demo mode toggle, enable alignment assistant in demo data

CAMPER_DEMO=1 only works when Xcode or simctl launches the process, so
a build installed locally and opened from the home screen had no way
to enable demo mode at all. Demo mode is now also a persisted toggle
under Settings > Entwicklung (Debug builds only), checked alongside the
environment variable. Takes effect after an app restart since
DeviceStore/BluetoothManager only read it at init.

Also mark the demo leveling sensor as live-connected - it had a level
reading but never a live link state, which left the Ausrichtungs-
Assistent button (and Live Activity toggle) disabled in demo mode.

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

CamperMonitor/Views/SettingsView.swift

@@ -4,12 +4,28 @@ import SwiftUI
 /// betrifft, steht bei diesem Gerät.
 struct SettingsView: View {
     @AppStorage(AppSettings.showDiagnosticsKey) private var showDiagnostics = false
+    #if DEBUG
+    @AppStorage(DemoData.enabledKey) private var demoModeEnabled = false
+    #endif
     @Environment(PhoneWatchLink.self) private var watch
     @Environment(\.dismiss) private var dismiss
 
     var body: some View {
         NavigationStack {
             List {
+                #if DEBUG
+                Section {
+                    Toggle("Demo-Modus", isOn: $demoModeEnabled)
+                } header: {
+                    Text("Entwicklung")
+                } footer: {
+                    Text("Füllt die App mit erfundenen Fahrzeugen und Messwerten, um "
+                         + "Ansichten ohne echte Sensoren zu prüfen. Wirkt erst nach "
+                         + "einem Neustart der App – einmal beenden (im App-Umschalter "
+                         + "nach oben wischen) und wieder öffnen.")
+                }
+                #endif
+
                 Section {
                     Toggle("Diagnose anzeigen", isOn: $showDiagnostics)
                 } header: {