Studio: persist external provider selection across page refresh (#5697)

Selecting a connected external provider (Anthropic, OpenAI, Google, etc.)
and refreshing the page reverted the picker back to no selection. Root
cause is that `PersistedInferenceParams` in `chat-settings-api.ts`
excludes `checkpoint` from the server-side settings payload by design.
Local model selections survive refresh because the backend re-derives
them from `/api/inference/status.active_model`, but external selections
have no backend mirror, so they were lost.

Fix: persist `external::*` checkpoints to a small dedicated
`localStorage` key (`unsloth_chat_last_external_checkpoint`) and hydrate
from it on store init. Local checkpoints continue to come from the
backend status as before; only external ids are mirrored client-side.
`setCheckpoint` writes the key when an external id is selected and
clears it when switching back to a local id, and `clearCheckpoint`
clears it so the picker does not snap back after an explicit reset.