fix(preview): prevent memory leak and CPU overhead in thumbnail generator by aborting video stream and adding timeout fallback

[AdityaNarayanPadhi]

Summary

This PR prevents memory leaks and background CPU overhead in the offscreen thumbnail generator caused by stalled seek operations when new videos are uploaded or components unmount (#1500).

Changes

• Updated cancelThumbnailRun in ThumbnailStrip.tsx to immediately set the source of the offscreen video element to "" and invoke load(). This halts active media loading/decoding instantly and clears occupied hardware/software decoder slots.
• Added a 2-second timeout fallback inside the offscreen canvas seek promise within generateThumbnails. If a seek stalls, the promise resolves and allows the loop to exit and clean up rather than hanging indefinitely.

Verification

• Uploading new videos rapidly in succession or unmounting the component during thumbnail generation now aborts active seeks immediately, resolving memory footprint creep and background thread usage.

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[github-actions]

👋 Thanks for your PR, @AdityaNarayanPadhi!

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