fix: prevent infinite export when looping background music

[Saylee12R]

Description

Added FFmpeg -shortest flag when a background music track is present.

This ensures exports terminate when the video stream ends, preventing excessively long or potentially infinite exports when:

• the source video has no audio track
• loop music is enabled
• the background music duration exceeds the video duration

Related Issue

Fixes #1493

Type of Contribution

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

Participant Info

• GitHub username: Saylee12R
• Contribution level: Beginner

Screen Recording

N/A (logic-only bug fix, no UI changes)

Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• No console.log statements left in
• This PR is related to a valid issue

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[github-actions]

👋 Thanks for your PR, @Saylee12R!

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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• Linked the issue this PR closes (e.g. Closes #123)
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[github-actions]

✅ PR Format Check Passed — @Saylee12R

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