feat: add Pillarbox Blur background option for Fit mode

[Kritika200520]

Description

This PR adds a Pillarbox Blur background option when resizing videos using the Fit (letterbox/pillarbox) mode. Instead of showing solid black bars in the empty letterbox space, users can now toggle a scaled-up, blurred version of the video playing in the background.

Key changes:

• Extended EditRecipe to support blurBackground state.
• Added a dynamic Pillarbox Blur toggle checkbox in the FramingControl component under Fit mode.
• Built a complex FFmpeg filter graph in buildVideoFilter using split, scale, crop, boxblur, and overlay to overlay the source video on top of a blurred background.
• Added unit tests in src/lib/tests/ffmpeg.test.ts to verify the generated filter strings for letterbox, blurred background, and crop scenarios.

Related Issue

Closes # [685]

Type of Contribution

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

Participant Info

• GitHub username: Kritika200520
• Contribution level (Beginner/Intermediate/Advanced): Intermediate

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

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[github-actions]

👋 Thanks for your PR, @Kritika200520!

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

• 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
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Happy coding! 🎉

[github-actions]

✅ PR Format Check Passed — @Kritika200520

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[vercel]

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Project	Deployment	Actions	Updated (UTC)
reframe	Error		May 20, 2026 2:34pm

[magic-peach]

@Kritika200520 the build is failing please fix it

[Kritika200520]

@magic-peach i did some changes please review it again