Migrate from C++ Qt to Rust and COSMIC

Migrate the Package Installer from C++ Qt to **Rust** using the **COSMIC** toolkit to align with modern Linux desktop standards and improve maintainability.

## Useful Links
- [libcosmic](https://github.com/pop-os/libcosmic) – UI toolkit
- [cosmic-app-template](https://github.com/pop-os/cosmic-app-template) – Reference implementation

## To do
- Set up Rust project structure using `libcosmic`
- Port core package management logic
- Implement COSMIC-native UI (responsive, dark/light theme support)
- Testing and release packaging