fix: Alle Rust-Warnings bereinigt

- unused variables: Underscore-Prefix (_real_path, _had_changes, _file_id)
- dead_code: #[allow(dead_code)] fuer zukuenftige Methoden
  (open_cloud_file, close_cloud_file, get_changes, LockResponse, SyncChangesResponse)

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

clients/desktop/src-tauri/src/sync/engine.rs

@@ -376,13 +376,14 @@ impl SyncEngine {
     }
 
     /// Open a .cloud placeholder file: download the real file, rename, return path
+    #[allow(dead_code)]
     pub async fn open_cloud_file(&self, cloud_path: &Path) -> Result<PathBuf, String> {
         let content = std::fs::read_to_string(cloud_path)
             .map_err(|e| format!("Platzhalter lesen: {}", e))?;
         let placeholder: CloudPlaceholder = serde_json::from_str(&content)
             .map_err(|e| format!("Platzhalter ungueltig: {}", e))?;
 
-        let real_path = cloud_path.with_extension("");
+        let _real_path = cloud_path.with_extension("");
         // Remove .cloud extension to get real filename
         let real_path = cloud_path.parent().unwrap().join(&placeholder.name);
 
@@ -399,6 +400,7 @@ impl SyncEngine {
     }
 
     /// Close a previously opened file: sync back, recreate .cloud, unlock
+    #[allow(dead_code)]
     pub async fn close_cloud_file(&self, real_path: &Path, file_id: i64) -> Result<(), String> {
         // Upload changes
         // We need the parent_id - for now upload to the same location