webclient: a dialog that answers which model is serving, and from where ...

The header only named a model when the route had fallen back, so "what is
running this session, on which host, with how much context" had no answer
in the UI. The context percentage doubles as the second way in: a session
bar is a fair place to ask what the bar measures.

GET /sessions/{id}/info reads in-process state only — the active route
entry, the health chain, the profile's effective window, run and client
counts — and the dialog polls it while open. /meta and the session head
now report the route's window instead of the global ollama_num_ctx, which
had been wrong for every cloud-backed profile.