Offer Expert-led setup from anywhere, not just onboarding

Having the Expert set something up for you is useful well past the first five minutes. Someone who came through onboarding and now wants a second thing built shouldn't have to do it by hand just because they're no longer new.

So it becomes a standing option rather than something that only exists during onboarding, sitting alongside Create Instance rather than replacing it.

The obvious first home is the team home empty state, which currently reads "It's looking a little empty. Create a Hosted Instance to get started." That's exactly the moment someone would take the offer.

### Done when

* The offer is reachable when the team already has applications and instances, not only when it's empty.
* Taking it starts an Expert conversation that provisions and builds the same way onboarding does.
* Creating things by hand is still a first-class path, not buried.

### Worth knowing

This changes the assumptions the rest of the epic runs on, which is the main thing to think about. Onboarding picks the instance type and version silently because a brand-new user has no basis for choosing. On an established team that's less obviously right: the person may well have an opinion, and getting it wrong costs them money rather than just being suboptimal.

Which pulls in flowfuse#8324, "Work out where the Expert should get instance pricing from". Silently picking an instance type for a trial is harmless. Doing it for a paying team without anyone seeing the cost is not, so that issue is effectively a dependency here in a way it isn't for onboarding.

Worth deciding whether this uses the same onboarding mode or a variant. Reusing it wholesale means silent defaults and no platform vocabulary for a user who already knows both, which may come across as evasive rather than helpful. A user who says "make me another dashboard instance, medium" should probably be taken at their word rather than have the Expert quietly decide.

There's a naming question too. "Onboarding" stops being the right word for it once it's a standing feature, and whatever the button says will shape how people think about it.