feat: add batch / multi-preset export support

[jaygaikar-09]

Description

Implemented the Batch / Multi-Preset Export feature for Reframe, allowing users to export the same video in multiple formats (YouTube, Instagram, TikTok, etc.) in a single run using FFmpeg.wasm.

This implementation follows all acceptance criteria mentioned in issue #668 and preserves the existing single-export workflow.

What was implemented

• Added a Batch Export mode toggle
• Added multi-select preset support using checkboxes
• Sequential export processing using a single FFmpeg.wasm worker
• Live export queue progress:
  • Export X of N
  • current exporting preset filename
• Individual download support for each generated file
• ZIP download support using fflate
• Graceful cancellation support during batch processing
• FFmpeg cleanup + memory reset between sequential exports
• Preserved backward compatibility with single export mode

Technical Enhancements

• Added reusable BatchExportPanel component
• Added batch progress + cancel state types
• Added abort + terminate handling for FFmpeg worker reset
• Added unique FFmpeg virtual filesystem paths per export
• Improved export lifecycle cleanup using finally
• Refactored queue management for sequential processing

Related Issue

Closes #668

Type of Contribution

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

Participant Info

• GitHub username: jaygaikar-09
• Contribution level (Beginner/Intermediate/Advanced): Advanced

Screen Recording

Recording / Loom link:

Video.Project.1.2.mp4

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]

👋 Thanks for your PR, @jaygaikar-09!

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

1. 🤖 Automated checks — build & TypeScript typecheck will run automatically
2. ✅ Vercel preview — a preview deployment will be created (requires maintainer authorization for fork PRs)
3. 👀 Code review — a maintainer will review your changes
4. 🚀 Merge — once approved, your PR will be merged!

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! 🎉

[github-actions]

✅ PR Format Check Passed — @jaygaikar-09

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.