docs: document frontend test commands

[gordonmurray]

Summary

• document the Node.js version that CI uses for frontend tests
• add the syntax and race-test commands
• clarify that the frontend tests need no package installation
• distinguish the single frontend job from the eight-version API matrix

Validation

• node --check web/vanilla/app.js
• node --test web/vanilla/tests/*.test.js — 5 passed
• git diff --check