fix: close waitlist modal on escape

[darad124]

Summary

• Add an Escape key handler while the Summer of AI waitlist modal is open
• Clean up the listener when the modal closes or the component unmounts

Closes #96

Validation

• npm run build
• Opened the local site, launched the waitlist modal, pressed Escape, and confirmed the modal closed

Notes

• npm run lint is currently blocked because the repo does not include an ESLint config file.

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