Add rendered preview to README generator

[sajeetharan]

Summary

• add a rendered preview tab beside Edit and Markdown source
• render GitHub-flavored Markdown and generated HTML with strict sanitization
• preserve profile cards, badges, tables, images, links, and mobile scrolling
• portal the README dialog above page overlays
• declare the existing twitter-api-v2 runtime dependency required by main

Validation

• npm test (190 passed)
• npm run build (passed)
• desktop/mobile browser checks passed
• injected script markup was removed and did not execute
• keyboard tab navigation and external link behavior verified
• git diff --check