fix: fixed audio filter chaining and add Vitest for unit testing

[Shivansh181003]

Fix: Handle extreme audio speeds via atempo filter chaining

Description

This pull request fixes the audio distortion and silent export failures that occur when the user sets an audio speed outside the [0.5, 2.0] range natively supported by the FFmpeg atempo filter.

To achieve arbitrary speeds like 0.25x or 4.0x, the buildAudioFilter function was refactored to dynamically chain multiple atempo filters together mathematically. Additionally, since the project lacked a unit test runner, vitest was added as a development dependency, and a dedicated test suite for ffmpeg.ts was implemented to protect this logic from future regressions.

Closes #6

Acceptance Criteria Details

• Valid atempo chain logic: Ensures mathematically that 0.25x speed is properly constructed as atempo=0.5,atempo=0.5.
• Unit tests added: A test suite (src/lib/tests/ffmpeg.test.ts) has been written using vitest to validate chaining logic across boundary values, standard use cases, and extreme speeds.
• Distortion-free exports: Export operations with speeds like 0.25x and 4.0x will now generate valid FFmpeg commands, preventing premature crashes and outputting correctly stretched audio.

PR Checklist

(Checked according to CONTRIBUTING.md guidelines)

• Code works in Chrome, Firefox, and Safari
• No new TypeScript errors (bunx tsc --noEmit)
• ESLint passes (bun run lint)
• UI changes tested on mobile (use browser DevTools) (N/A)
• Accessibility: new interactive elements have ARIA labels (N/A)
• Issue number referenced in PR description

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[github-actions]

👋 Thanks for your PR, @Shivansh181003!

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[github-actions]

✅ PR Format Check Passed — @Shivansh181003

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[magic-peach]

Hey @Shivansh181003! This PR has a merge conflict that needs to be resolved before it can be reviewed. Please rebase onto the latest main:

git fetch origin
git rebase origin/main
# resolve any conflicts
git push --force-with-lease

Also note: this PR adds Vitest as a testing dependency. Please check that it's added to bun.lock (not package-lock.json) and that the tests run correctly with bun test.

[magic-peach]

Hi @Shivansh181003 👋 This PR has a merge conflict with main. Please update your branch:

git fetch upstream && git merge upstream/main
# resolve any conflicts, then:
git push

Once conflicts are resolved and CI passes, this will be ready to merge. 🙂

[Shivansh181003]

I have resolved the conflicts.
While on the issue is closed by you, does it needs any further actions or i also should consider it as finished ?
@magic-peach

[magic-peach]

Hey @Shivansh181003, the audio filter chaining fix and Vitest tests look good! Quick note: the PR adds vitest to bun.lock and package.json, which is fine. The only concern is that the tests are in src/lib/tests/ rather than the conventional src/lib/__tests__/ directory — but that's minor. Could you also check if there's a vitest config needed or if bun test runs these? Overall looks solid!