Desktop: add Developer ID signing and notarization for stable DMG

[justn-hyeok]

Context

CodeAgora v0.1.0 shipped the macOS arm64 Desktop DMG as an unsigned preview. The release explicitly does not claim Developer ID signing, notarization, or stapling.

Acceptance criteria

• macOS arm64 DMG is Developer ID signed.
• Notarization succeeds and the DMG/app is stapled.
• Gatekeeper launch path is documented and verified on a clean macOS machine.
• Release evidence records signing identity, notarization status, stapling status, artifact SHA, and verification commands.
• Public docs and release notes stop describing Desktop as unsigned preview only after fresh evidence is attached.