ui : load a model on the backend that serves it
The status store pinned the active backend to the local server before
loading, and the service posted to a relative /models/load, so a model
from an external llama-server was loaded on the server serving this UI.
Load and unload now name the backend the model belongs to.
Assisted-by: pi:llama.cpp/DeepSeek-V4.1-Flash
