[codex] Improve desktop PDF reader

[OmGuptaIND]

Summary

Replaces the desktop PDF preview native Chromium embed with an app-native PDF.js reader so the bulky thumbnail/sidebar strip is removed and the preview has controlled, compact reader UI.

What changed

• Added a PDF.js-backed PdfRenderer with page navigation, direct page jump, zoom, fit-width/fit-page, rotate, download, lazy page rendering, and a text layer for selection.
• Added reader styling for centered pages, compact controls, responsive toolbar behavior, and per-page loading/error overlays.
• Fixed the document preview loading state that spread the spinner and label far apart by restoring a centered flex layout for DOCX/PDF document loading overlays.
• Added the desktop pdfjs-dist dependency pinned to a Node 20-compatible release.

Validation

• pnpm --filter @anton/protocol build
• pnpm exec biome check packages/desktop/src/index.css packages/desktop/src/components/artifacts/PdfRenderer.tsx packages/desktop/package.json
• pnpm --filter @anton/desktop typecheck
• pnpm --filter @anton/desktop build

Build succeeds. It still prints the pre-existing CSS minifier warnings for escaped file:* selectors and the existing chunk-size warning.