Migrate from GTK 3 to COSMIC

[azdanov]

Migrate the welcome application from Rust GTK 3 to libcosmic to align with modern Linux desktop standards and improve maintainability.

Useful Links

• libcosmic
• cosmic-app-template

To do

• Set up libcosmic for use
• Port core welcome app logic
• Implement COSMIC-native UI (responsive, dark/light theme support)
• Testing and release packaging