feat: enhance PromptKey with single instance support and update README

- Added `tauri-plugin-single-instance` to ensure only one instance of the app runs at a time.
- Updated `main.rs` to handle new instance focus and window management.
- Enhanced README with improved structure, features, and installation instructions.
- Removed the batch script for starting the application as it is no longer needed.
- Updated dependencies in `Cargo.toml` for better compatibility.

Author: Haaaiawd