Migrate from proxifyImageUrl to proxifyImageSrc #777

Replace old proxifyImageUrl function with newer proxifyImageSrc
that uses Base58 encoding for image URLs.

Changes:
- Update card.tsx, select-image-item.tsx, post-img.tsx, renderer.ts
- Delete old-profixy.ts (no longer needed)
- Fix URL encoding for special characters
- Use TextEncoder for browser compatibility instead of Buffer
- Add index signature to ProxyOptions for querystring.stringify

Closes #777

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>