Make asset compilation part of the build_wheel hatchling build_hook

[potiuk]

Currently, we have custom build hook in hatchling that performs two things:

• retrieval of .git commit (needs git to be available on path)
• compiling www-assets (needs pre-commit to be available on path)

We are not using npm but pnpm in airflow ui - new Airflow 3.0 UI and we are going to drop the old www - npm driven environment. PNPM is way more portable and easy to install and 2x faster than npm so we could potentially move both git and asset compilation to build_wheel build hook, so that asset compilation happens always when airlfow is installed for editable installation or when wheel is built.

We could install pnpm via https://pnpm.io/installation#on-posix-systems and do the git bit might be optionally skipped if git is not available.

This way the contributors will not have to worry about compiling assets when installing airlfow locally for development or building wheels - the compilation would happen automatically when pip install -e . or uv sync or and PEP compliant "build wheel` happens.

This has the drawback of longer first-time installation and needs a bit smart determination on how to inform that the package needs to be reinstalled / buiild_hook run during the development.