- Enable `calendar.enable` instead of `evolution.enable` in all host
system‑modules files.
- Add `modules/core/calendar.nix` defining the calendar option and
installing GNOME Calendar and Contacts.
- Remove the deprecated `modules/core/evolution.nix` and update
`core/default.nix` imports accordingly.
- Include the GNOME package set via `./gnome.nix` in
`modules/core/packages/default.nix`.
