feat: oauth add passkey after signup (popup flow)

[linnall]

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)
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• 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 introduces a new user signup event in the AlchemySigner and updates various components to handle this event, alongside some refactoring in the useAuthModal and related hooks.

Detailed summary

• Added newUserSignup event to AlchemySignerClientEvents.
• Emitted newUserSignup on successful user signup in AlchemySignerWebClient.
• Refactored closeAuthModal in useAuthModal to improve readability.
• Updated CompletingOAuth to simplify auth step setting.
• Adjusted AuthModal to handle signup logic with useNewUserSignup.

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[moldy530]

do we need to update the ui-demo app to default back to using pop-up now?

[linnall]

do we need to update the ui-demo app to default back to using pop-up now?

done in #1140 before merging (here and here)