Browsers only honor a transparent same-origin iframe canvas when the
embedder and the iframe agree on color-scheme; the neode-ui shell's
:root{color-scheme:dark} was forcing the AIUI frame opaque and hiding
the background art behind the chat.
Also adds neode-ui/vite.preview.config.mts (preview on :8100 against
the mock backend on :5959).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
