feat: local backup (#5054)

* feat: local backup

* refactor(backup): make local backup helpers synchronous and clean up redundant checks

- Converted local backup helpers to synchronous functions to remove unused async warnings and align command signatures.
- Updated list/delete/export commands to call the sync feature functions directly without awaits while preserving behavior.
- Simplified destination directory creation to always ensure parent folders exist without redundant checks, satisfying Clippy.

src-tauri/src/cmd/mod.rs

@@ -4,6 +4,7 @@ pub type CmdResult<T = ()> = Result<T, String>;
 
 // Command modules
 pub mod app;
+pub mod backup;
 pub mod clash;
 pub mod lightweight;
 pub mod media_unlock_checker;
@@ -21,6 +22,7 @@ pub mod webdav;
 
 // Re-export all command functions for backwards compatibility
 pub use app::*;
+pub use backup::*;
 pub use clash::*;
 pub use lightweight::*;
 pub use media_unlock_checker::*;