feat: prototype the auth modal hook

[moldy530]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
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PR-Codex overview

This PR focuses on adding a new color variant fg-accent-brand, updating theme colors, modifying button styles, and introducing an AuthModal component with authentication types.

Detailed summary

• Added fg-accent-brand color variant
• Updated theme colors with fg-accent-brand
• Modified button styles for primary, secondary, and authentication buttons
• Introduced AuthModal component with authentication types and functionality

The following files were skipped due to too many changes: examples/ui-demo/src/app/page.tsx

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[moldy530]

[dphilipson]

Looks good!