Update User Onboarding State

Partial update merged into the stored state.

Scalars overwrite when supplied; list entries are unioned into the stored lists, so concurrent updates (e.g. two tabs marking different tooltips) don't drop each other's items.

Per-user onboarding state, stored under UserConfigurationKey.ONBOARDING.

Server-authoritative replacement for the browser-localStorage onboarding store, so the post-signup gate and one-time tooltips hold across devices.