fix: add dismissible large file warning banner for files over 500MB

[AKIB2005]

Description

When a user uploads a video file larger than 500MB, the app would silently
hang or crash the browser tab during export with no feedback. This PR adds
a visible, dismissible warning banner that appears immediately after a large
file is selected, informing the user of the risk before they attempt export.

Changes made:

• Replaced the old basic 100MB plain text warning in VideoEditor.tsx with
  a proper yellow alert banner triggered at 500MB
• Banner displays the exact file size in MB
• Banner is dismissible via a ✕ button
• Warning resets automatically when a new file is selected
• Uses AlertTriangle from lucide-react (already in project, zero new dependencies)

Related Issue

Fixes #(Closes #1563)

Type of Contribution

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

Participant Info

• GitHub username: AKIB2005
• Contribution level (Beginner/Intermediate/Advanced): Beginner

Screen Recording

Recording / Loom link: (record yourself uploading a 500MB+ file and showing the banner appears, then dismissing it)

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)

[vercel]

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[github-actions]

✅ PR Format Check Passed — @AKIB2005

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, @AKIB2005!

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

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

What happens next

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Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
• Build passes (bun run build)

Useful links

• 📖 CONTRIBUTING.md
• 🐛 Open Issues
• ⭐ Star the repo to support the project!

Happy coding! 🎉