Document macOS build instructions

[TheNorthStarYang]

Goal

Document the current macOS build flow for the AhaKey Desktop baseline.

Scope

• describe the build entry points
• explain current prerequisites
• mention known packaging/signing limitations
• avoid unrelated refactors

Where to edit

• repository: desktop
• relevant path: platforms/macos/client/README.md

Notes

This issue is part of the early community onboarding / baseline cleanup.

Contributors are welcome.