fix: trap keyboard focus within onboarding dialog

[karthikeya20012007]

Description

Fixes an accessibility issue where keyboard focus could escape the onboarding dialog while the tour was active.

Added FocusTrap to keep keyboard navigation contained within the onboarding modal and preserved existing dismissal/navigation behavior.

Related Issue

Closes #878

Type of Contribution

• Bug fix
• New feature
• Documentation update
• GSSoC contribution

Participant Info

• GitHub username: karthikeya20012007
• Contribution level (Beginner/Intermediate/Advanced): Intermediate

Screen Recording

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Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (required for UI/feature/design changes)

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[github-actions]

👋 Thanks for your PR, @karthikeya20012007!

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🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

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• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
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