Studio: also persist external checkpoint when picker calls setParams (#5700)

The first pass only wired the localStorage mirror into `setCheckpoint`,
but the main chat-page picker actually selects an external model by
calling `setParams({ ...store.params, checkpoint: value })`. That path
never hit `setCheckpoint`, so the persisted slot stayed empty and a
refresh fell back to whatever `/api/inference/status.active_model`
returned -- the previously loaded local model (Qwen3.5 etc) or null
("Select model") when nothing was loaded locally.

Mirror the persistence in `setParams` whenever the checkpoint changes
so every entry point converges on the same behavior. `setCheckpoint`
still does it directly so the load path (compare, GGUF auto-load,
gemma fallback in chat-adapter) keeps working.