fix: skip runtime rebuild when esbuild is omitted

[shyn]

Problem

pi install / pi update of this git package runs npm install --omit=dev. That skips esbuild (a devDependency) but still runs prepare → build:runtime, which imported esbuild unconditionally:

Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'esbuild' imported from
  .../pi-generative-ui/.pi/extensions/generative-ui/build.mjs
Error: npm install --omit=dev failed with code 1

The page-side bundle is already committed (runtime.bundle.ts), and the README says end users should not need a build step.

Fix

If esbuild is not installed and runtime.bundle.ts already exists, skip the rebuild. Developers with a full install still regenerate the bundle as before. A missing bundle without esbuild still fails loudly.

This is an alternative to #6 (moving esbuild into dependencies). Shipping esbuild to every consumer is unnecessary when the committed bundle is the artifact they actually use.

Tested

npm install --omit=dev
# prepare: esbuild not installed; using committed runtime.bundle.ts
# exit 0