COSMIC Desktop Print Dialog

Build a production-ready CPDB-based print dialog for COSMIC and integrate it with xdg-desktop-portal-cosmic (PreparePrint + Print) so desktop and sandboxed apps can print through a native COSMIC UX. The project also defines an async event-driven backend model in cpdb-rs (aligned with zbus and libcosmic/iced) to bridge async CPDB callbacks cleanly into libcosmic/iced state updates, with handling for printer discovery, options, translations, job submission, and failure scenarios.