Refactor: Extract platform-specific roles to base-linux and base-darwin

Create base-linux and base-darwin modules to cleanly separate platform-
specific role imports from shared roles. This prevents importing modules
that require platform-specific home-manager modules (like plasma-manager
on NixOS) in environments where they don't exist (like nix-darwin).

- base-linux includes: plasma-manager, i3+sway
- base-darwin includes: aerospace
- roles/default.nix now only contains truly cross-platform roles

This architecture makes it immediately clear which roles are shared
versus platform-specific and makes it easy to add new platform-specific
roles in the future.

home/home-desktop.nix

@@ -29,5 +29,6 @@
 
   imports = [
     ./roles
+    ./roles/base-linux
   ];
 }