Add MacBook Air (mba) darwin configuration

- Add darwinConfigurations."mba" entry using same config as iMac
- Both machines now use identical system configuration
- Ready for machine-specific customizations in the future

Usage:
- iMac: darwin-rebuild switch --flake .#iMac
- MacBook Air: darwin-rebuild switch --flake .#mba

flake.nix

@@ -60,6 +60,10 @@
       modules = [ configuration ];
     };
 
+    darwinConfigurations."mba" = nix-darwin.lib.darwinSystem {
+      modules = [ configuration ];
+    };
+
     homeConfigurations.yanlin = home-manager.lib.homeManagerConfiguration {
       pkgs = nixpkgs.legacyPackages.aarch64-darwin;
       modules = [ homeConfiguration ];