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/calendar.nix

@@ -0,0 +1,16 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}: {
+  options.calendar.enable = lib.mkEnableOption "Enable GNOME Calendar";
+
+  config = {
+    services.gnome.evolution-data-server.enable = lib.mkIf config.calendar.enable true;
+    environment.systemPackages = lib.mkIf config.calendar.enable (with pkgs; [
+      gnome-calendar
+      gnome-contacts
+    ]);
+  };
+}