Better preview rendering and crate structure refactor

I'm working on a refactor of the crate structure and significantly improved preview rendering at https://github.com/fu5ha/pi-apply-patch/pull/4

It's not as big as the diff line says because a lot of that is inconsequential formatting changes in package-lock.json and other config files from a biome fmt run, but it is still a large change from the code here.

Let me know if you're interested in me PRing this or not.

Review the linked pull request #4 first, along with the crate structure and preview-rendering changes it describes. Check the package-lock.json and other configuration formatting changes separately from the substantive code; the work is done when the refactor and improved preview rendering are incorporated or the proposed scope is clarified.

Written by the indexing model from the issue text.