fix(react-ui): share single /api/operations poller across consumers (#10029)

useOperations() spun up its own setInterval per hook instance, so on
pages like /app/models the OperationsBar in App.jsx plus the page's
own useOperations() call each polled /api/operations at 1 Hz - 2 RPS
sustained for the whole session, repeated on Backends and Chat.

Lift the poller into an OperationsProvider mounted under AuthProvider
so all consumers (OperationsBar, Models, Backends, Chat) share one
timer. The hook file re-exports from the context to keep call sites
unchanged.


Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>