v2.55.3 — Code-Review-Fixes: foregroundServiceType (Android 14+), POST_NOTIFICATIONS-Check, FG-Service-Notification-Channel, try/catch _zeigeSyncNotification

CRITICAL:
- AndroidManifest: foregroundServiceType="dataSync" für BackgroundService (Android 14+ MissingForegroundServiceTypeException)
- main.dart: notificationChannelId + initialNotificationTitle + foregroundServiceTypes für FlutterBackgroundService (fehlende FG-Notification → ANR/crash nach 5s)

MEDIUM:
- sync_service.dart: POST_NOTIFICATIONS-Permission-Check vor _starteForegroundService (Android 13+)
- sync_service.dart: try/catch um _zeigeSyncNotification (plugin-dispose-Sicherheit)

flutter analyze: clean | flutter test: 194/194 passed

android/app/src/main/AndroidManifest.xml

@@ -36,6 +36,13 @@
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
         </activity>
+        <!-- v2.55.3: Foreground-Service-Type für Android 14+ (compileSdk 36).
+             Ohne dieses Attribut crasht der Service-Start mit
+             MissingForegroundServiceTypeException. -->
+        <service
+            android:name="id.flutter.flutter_background_service.BackgroundService"
+            android:foregroundServiceType="dataSync" />
+
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
         <meta-data