feat(web): make brain onboarding the default flow
- Replace legacy /onboarding with the brain onboarding flow (about -> sources -> ingest -> team -> done)
- Add brain-home dashboard page and widgets
- Old /onboarding/brain deep links fall through to the existing catch-all redirect
feat(onboarding): create org on About step + remove skip
- Create the organization on About -> Continue (authClient.organization.create + setActiveOrg), persisting brain context to org metadata; fixes the no-org redirect loop
- Show a Creating… state on Continue while the org is created
- Remove Skip on the required About step (name + workspace)
- Add generateOrgSlug/generateUsername helpers to onboarding-brain/types
