Wrote a similar PoC here: https://github.com/s1liconcow/autoapp

Some ideas - use a slower 'design' model at startup to generate the initial app theme and DB schema and a 'fast' model for responses. I tried a version using PostREST so the logic was in entirely in the DB and but then it gets too complicated and either the design model failed to one-shot a valid schema or the fast model kept on generating invalid queries.

I also use some well known CSS libraries and remember previous pages to maintain some UI consistency.

It could be an interesting benchmark or "App Bench". How well can an LLM one-shot create a working application.