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/roles/base-darwin/default.nix

@@ -0,0 +1,7 @@
+{
+  # Base imports for Darwin home configurations
+  # Includes Darwin-specific roles that only work on macOS
+  imports = [
+    ../aerospace
+  ];
+}