fix: Rust MutexGuard ueber await - delta_sync Send-Fehler

MutexGuard wird jetzt vor dem .await gedroppt (take + put back),
damit der Future Send-kompatibel ist wie Tauri es erfordert.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/desktop/src-tauri/src/lib.rs

@@ -59,9 +59,15 @@ async fn start_sync(state: State<'_, AppState>) -> Result<Vec<String>, String> {
 
 #[tauri::command]
 async fn delta_sync(state: State<'_, AppState>) -> Result<Vec<String>, String> {
-    let mut engine_guard = state.sync_engine.lock().unwrap();
-    let engine = engine_guard.as_mut().ok_or("Sync nicht gestartet")?;
-    engine.delta_sync().await
+    // Extract engine from state, dropping the MutexGuard before .await
+    let mut engine = {
+        let mut guard = state.sync_engine.lock().unwrap();
+        guard.take().ok_or("Sync nicht gestartet")?
+    };
+    let result = engine.delta_sync().await;
+    // Put engine back
+    *state.sync_engine.lock().unwrap() = Some(engine);
+    result
 }
 
 #[tauri::command]