Static site build copies stylesheet sources next to output.css

[dubadub]

Found during review of #456. Pre-existing on main (it shipped styles.css and custom-styles.css the same way).

copy_static_assets in src/build/writer.rs copies the whole embedded static/ tree, so the generated site contains static/css/input.css and static/css/components.css even though pages only link output.css and cooking-mode.css.

Fix: an exclude list in the writer (or a separate embed folder for shipped assets).