Add macOS menu bar spacing configuration

- Create system/ directory for system-level nix-darwin configurations
- Add system/macos-defaults.nix with NSStatusItemSpacing and NSStatusItemSelectionPadding settings
- Configure optimal spacing (6) and padding (12) values for menu bar items
- Use CustomUserPreferences to support options not yet in nix-darwin main
- Set system.primaryUser to support user-specific preferences

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -15,6 +15,10 @@
   outputs = inputs@{ self, nix-darwin, nixpkgs, home-manager, nixvim, claude-code }:
   let
     configuration = { pkgs, ... }: {
+      imports = [
+        ./system
+      ];
+
       environment.systemPackages =
         [ pkgs.vim
           pkgs.git