Switch from Evolution to GNOME Calendar module

- 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`.

modules/core/default.nix

@@ -2,8 +2,8 @@
   imports = [
     ./packages
     ./boot.nix
+    ./calendar.nix
     ./docker.nix
-    ./evolution.nix
     ./flatpak.nix
     ./fonts.nix
     ./garandos-tui.nix