chore(main): release 4.21.0

[github-actions]

🤖 I have created a release beepboop

4.21.0 (2025-09-10)

Features

• add zsh shell detection to install script (#1539) (50ea2a3)
• api: determine if docker container has update (#1582) (e57d81e)

Bug Fixes

• white on white login text (ae4d3ec)

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[github-actions]

🤖 Created releases:

• v4.21.0

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