Epic: Onboarding Flow Improvements

The current onboarding stops at "you have created a flag" — users never reach the SDK install or a first evaluation in-app.

Following the audit and team review, we're collapsing onboarding to a fast path from signup to a first working flag, with a role question at the start so different personas get a path that fits their job.

**Done when**

The new flow is live behind a Flagsmith-on-Flagsmith flag, measured against the existing flow, with a recorded decision to ramp to 100% or roll back.