Fix image converter large-file worker path

[oguzhankir]

Summary

• Fix the Image Format Converter large-input Web Worker path by posting a structured-clone-safe plain request object instead of Svelte reactive state.
• Clarify converter stat cards so source/output cards show actual original and converted sizes.
• Add regression coverage for worker request structured cloning.

Type of change

• feat
• fix
• docs
• refactor
• test
• chore

Scope

• JSON
• YAML
• XML
• CSV
• TOML
• Text
• Encode/Decode
• Shared UI
• Infrastructure / CI
• Documentation

Checklist

• I read CONTRIBUTING.md and .windsurfrules
• I tested locally with pnpm test
• I ran pnpm check
• I ran pnpm lint
• I ran pnpm build
• I added/updated tests for changed behavior
• I added/updated i18n keys for all locales (en,tr,de,es,fr,it) if UI text changed
• I used dynamic import() for heavy processing libraries
• I kept user data processing in-browser only
• I linked the related issue (or explained why none is needed)

Screenshots / Demo (if UI changed)

Large-input worker validation:

large_image_converter_worker_stats_demo.mp4

Related issue

No linked issue; reported directly by the user.

Notes for reviewers

• This is not a Cloudflare Worker issue; the fixed worker is a browser Web Worker.
• Large file requests now clone safely before postMessage(), avoiding the previous worker encoding failure path.

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