fix(desktop): apply the configured cursor theme to the Hyprland session

home.sessionVariables (XCURSOR_THEME/XCURSOR_SIZE from home.pointerCursor)
land in hm-session-vars.sh, which only a login shell sources. UWSM starts
the graphical session without it, so Hyprland never saw the theme and drew
its built-in cursor instead of the configured Bibata-Modern-Ice.

Declare the cursor through Hyprland's own env directive, read at compositor
startup regardless of the shell profile, sourced from config.stylix.cursor
so the theme and size stay defined in one place.

modules/desktop/theming.nix

@@ -34,6 +34,8 @@ in
 
       cursor = {
         package = pkgs.bibata-cursors;
+        # Solid white with a dark outline, so it stays easy to spot against the
+        # dark Nord wallpaper.
         name = "Bibata-Modern-Ice";
         size = 24;
       };