Use es modules

[eumalin]

I saw that the main js file was growing too big. It made more sense to split it into es modules with help from AI.

Split print-holdings-checker.js into three modules under src/js/print-holdings-checker/: validate.js has the type definitions and validation logic, ui.js has the card and error rendering, and index.js is the entry point that wires everything together.

In local dev WordPress serves the raw index.js as a native ES module, so I added a type="module" case to the existing add_type_attribute filter in functions.php (local env only). For deploy, esbuild now bundles the modules into the same dist/js/print-holdings-checker.min.js it always produced

The old print-holdings-checker.js is gone, and watch_build.js and package.json both point to the new entry point with --bundle added.

[carylwyatt]

This looks good! Thanks for the explanation, it made it easy for me to know exactly what I was looking at.