fix: prevent accidental navigation during video export

[Ram-sah19]

Description

This PR fixes the issue where refreshing or closing the browser tab during an active video export would immediately cancel the export process without warning.

Changes Made

• Updated the beforeunload event listener in useVideoEditor.ts

• Added e.returnValue = "" to ensure modern browsers trigger the native confirmation dialog

• Refined the shouldWarn condition to activate only during:

  • loading-engine
  • exporting

• Removed the done status from the warning condition so the event listener detaches correctly after a successful export

These changes improve export reliability and help prevent accidental interruption during long-running video exports.

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Related Issue

Closes #740

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Type of Contribution

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

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Participant Info

• GitHub username: @Ram-sah19
• Contribution level: Intermediate

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Screenshot

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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

[vercel]

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[github-actions]

✅ PR Format Check Passed — @Ram-sah19

Basic format checks passed. A maintainer will review your code changes.

This does not mean the PR is approved — it just means the format is correct.

[github-actions]

👋 Thanks for your PR, @Ram-sah19!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

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

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Quick checklist

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• Linked the issue this PR closes (e.g. Closes #123)
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