feat: add session export history

[saurabhhhcodes]

Summary

• add a session-only export history list for the five most recent completed exports
• keep each history item downloadable with filename, size, format, and timestamp metadata
• store separate object URLs for history downloads so current-result cleanup does not break re-downloads

Validation

• git diff --check
• npx tsc --noEmit --pretty false
• npm run lint -- --file src/hooks/useVideoEditor.ts --file src/components/DownloadResult.tsx --file src/components/VideoEditor.tsx --file src/lib/types.ts
• npm run build

Fixes #105

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[github-actions]

👋 Thanks for your PR, @saurabhhhcodes!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

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Quick checklist

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• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
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