fix: abort ffmpeg asset downloads on cancel

[Sammmyyyyyyy]

Description

Implements proper cancellation support for FFmpeg core asset loading inside the worker pipeline.

Previously, cancelling export/loading only stopped the UI flow while large FFmpeg assets (ffmpeg-core.js, ffmpeg-core.wasm, and ffmpeg-core.worker.js) continued downloading in the background without interruption.

This PR propagates AbortSignal support through all FFmpeg asset fetches and worker loading flows to ensure downloads are immediately terminated when a cancel action is triggered.

Related Issue

Closes #1247

Type of Contribution

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

Participant Info

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

##Changes Made

• Added AbortSignal support to fetchWithIntegrity
• Propagated abort signals through loadCore
• Added dedicated worker-level AbortController handling
• Added cancellation handling for:
  • ffmpeg-core.js
  • ffmpeg-core.wasm
    -ffmpeg-core.worker.js
• Added safer AbortError handling during load/export cancellation
• Prevented stale FFmpeg initialization after cancellation events

##Impact

• Stops unnecessary ~32 MB background downloads immediately on cancel
• Prevents wasted bandwidth usage on slow or metered connections
• Avoids stale worker state and resource leaks during interrupted export sessions

Recording / Loom link: ## 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)

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[github-actions]

👋 Thanks for your PR, @Sammmyyyyyyy!

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[github-actions]

✅ PR Format Check Passed — @Sammmyyyyyyy

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