fix(init): run frontend npm install with prefix first

[iroiro147]

Summary

• change generated web-frontend build files to run npm --prefix frontend install
• update checked-in frontend examples to use the same prefix-first install command
• update scaffold README/test coverage and add a changelog fragment

Tests

• zig build test-tooling
• scripts/gate.sh fast origin/main
• git diff --check

Refs #58. Related #53.

[vercel]

@iroiro147 is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.