fix: enforce -t flag on export to prevent infinite loop with backgrou…

[shahabhilash]

Description

Fixes an issue where adding a looped background music track to a silent video at 1x speed causes FFmpeg to export indefinitely.

By enforcing the -t duration flag on all exports (rather than just those where speed != 1), FFmpeg is strictly told exactly when to stop encoding, preventing infinite loops caused by -stream_loop -1 combined with no native audio track.

Related Issue

Fixes #1577

Type of Contribution

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

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)
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above

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[github-actions]

👋 Thanks for your PR, @shahabhilash!

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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[github-actions]

✅ PR Format Check Passed — @shahabhilash

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[magic-peach]

Retargeted this from #1284 to #1577 — #1284 was just closed as a duplicate of #1577, which tracks the same infinite-export-loop bug and is now the open survivor issue. Your fix itself looks solid and unaffected by that change.

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