Always give the user a way out of the onboarding page

During onboarding there is no navigation and no way to sign out of FlowFuse until the flow completes; a user who stalls mid-flow is stuck in the app.

- Set the onboarding flag to false as soon as the agent moves the onboarding phase to done.
- Set it to false as well when the user clicks "Set it up myself".
- Show the user avatar menu on the onboarding page, to the right of "Set it up myself", so the user can sign out of FlowFuse. Sign-out is the only navigation offered there; "Set it up myself" remains the way into the app.

Done when: the user can always sign out of FlowFuse from the onboarding page, and completion (by agent or by skip) restores normal navigation.