ui : polish the shell
The secondary button gets its own look back and the chat add button its
own light surface. Pressable elements get a pointer cursor again, the
font rendering smooths on the app shell, and the agent skills stay out
of prettier's way.
The root layout props probe rework that came with this polish reads the
providers api url and stays with the providers change instead.
Assisted-by: pi:llama.cpp/DeepSeek-V4.1-Flash
