Add macOS alertDialog via NSAlert

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Summary

• Expose alertDialog(title, message) on macOS using AppKit NSAlert, matching the existing Windows API.
• Add NSAlert bindings in macdefs.nim and a small examples/alertdialog.nim smoke example.

Test plan

• nim r examples/alertdialog.nim on macOS shows a native alert and continues after OK
• Confirm Windows alertDialog still builds/unchanged
• Optional: call alertDialog from an app after Steam/sound init failure on Mac

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