Enable VirtualBox and add C# Omnisharp support

Update ChatGPT desktop entry icon SHA256 to match new asset.

hosts/Garand-Laptop/system-modules.nix

@@ -3,7 +3,7 @@ _: {
   Container & Packaging
   */
   docker.enable = true; # Docker: container runtime and management
-  virtualbox.enable = false; # VirtualBox: PC emulator
+  virtualbox.enable = true; # VirtualBox: PC emulator
   virualization.enable = false; # GUI application for managing local and remote virtual machines through libvirt
   flatpak = {
     enable = true; # Flatpak: universal packaging system for Linux