Expert starts writing flows before the immersive editor has finished loading

Came up in the Expert demo. When the Expert opens an instance in the immersive editor and then writes flows into it in the same turn, the flow writes race the editor coming up and fail.

`ui_navigate` reports `{ success: true }` as soon as the Vue route resolves, but the editor behind it is still booting at that point, so the flow building tools fire at something that isn't ready yet.

Today the only thing between the two steps is a line in the `ui_navigate` tool description telling the model to wait a few seconds and retry if it looks wrong. That's a hint to the model, not a guarantee, and in the demo it lost the race.

Might be worth having navigation not report back until the editor is actually ready to take edits, so the model doesn't have to guess. A plain cooldown after navigating would also do it, though it's more of a band-aid.

Repro: ask the Expert to open an instance and add some nodes to it in one go.

Relevant: `frontend/src/mcp/tools/navigation.ts`