chore: remove legacy js/ directory superseded by src/ TypeScript port

The pre-TypeScript JavaScript implementation under js/ is no longer
referenced by index.html, sql2er.html, vite.config.ts, or any CI
workflow. The active app is built from src/ via Vite.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ystemsrx