ui : model download pipeline (#27959)

* ui : model download pipeline

Track HuggingFace downloads end to end: the server download/cancel endpoints,
a status manager fed by the /models/sse download progress events, and a
models-discover store holding the catalog and detail state for the discover
view. Downloaded and in-flight entries are excluded from the loadable model
list.

Assisted-by: pi:GLM-5.3-Flash

* ui : route sidecar tag lookup through the sidecars util, validate the paused list

Assisted-by: pi:zai-org/GLM-5.3-Flash