Fix classic CAD worker boot and show loading notice

[SmartAI]

Summary

• keep the Pyodide worker valid as a classic worker in Vite dev mode
• show a centered canvas notice while the CAD environment downloads or installs
• add regression coverage for classic-worker imports and version synchronization

Root cause

The Pyodide CDN was healthy. A runtime import added to the classic worker caused Vite dev mode to serve syntax that classic workers cannot parse, leaving the UI at its initial downloading status before any runtime request started.

Verification

• npm test
• npm run build
• Chromium cold boot reaches Ready with all Pyodide runtime requests returning 200
• Chromium reload reaches Ready with zero console errors