refactor: move optional modules to modules/optional/

- Move virtualbox-guest.nix and vscode-server.nix from modules/server/ to modules/optional/
- Create modules/optional/README.md with usage docs
- Update profiles/virtualbox.nix and profiles/wsl.nix import paths
- Update modules/server/README.md to reflect removal
- Update root README.md directory tree with optional/ entry

profiles/virtualbox.nix

@@ -4,7 +4,7 @@
     ../modules/core
     ../modules/desktop
     ../modules/server/ssh.nix
-    ../modules/server/virtualbox-guest.nix
-    ../modules/server/vscode-server.nix
+    ../modules/optional/virtualbox-guest.nix
+    ../modules/optional/vscode-server.nix
   ];
 }

profiles/wsl.nix

@@ -3,7 +3,7 @@
   imports = [
     ../modules/core
     ../modules/server/ssh.nix
-    ../modules/server/vscode-server.nix
+    ../modules/optional/vscode-server.nix
   ];
 }