phsl / new-api

* fix(playground): prevent model group option stretching

- keep group options at a fixed 2rem height and align them to the top.
- organize layout code in a dedicated module and cover layout and scrolling behavior.

* docs(web): strengthen frontend testing requirements

- require regression coverage for behavior changes, bug fixes, and UI states.
- define module-scoped test organization, stable assertions, mocks, and verification rules.