atomic write for deleted-sheet backups

fs::write left the one safety-net copy of a deleted sheet exposed to
mid-write corruption; use the existing temp+rename helper like every
other user-data write.

src-tauri/src/main.rs

@@ -611,7 +611,7 @@ fn backup_deleted_sheet(
         safe,
         chrono::Local::now().format("%Y-%m-%d-%H%M%S")
     );
-    fs::write(bin.join(name), contents).map_err(|e| e.to_string())
+    write_atomic(&bin.join(name), &contents)
 }
 
 #[tauri::command]