Tier 2: Onboarding flow → user model (app-side 5-phase chat)

[CodeCreatorManMike]

What to build

The onboarding chat (app/onboarding.tsx) already exists but needs to wire into the new user model:

• Phase 1: Name + timezone
• Phase 2: Primary 6-month goal
• Phase 3: Non-negotiables (max 5 daily habits)
• Phase 4: Feedback preference + rest days
• Phase 5: Study/work context (for document upload prompt)
• After each phase, backend silently extracts structured profile data
• On completion: redirect to home, trigger first layout generation

Must call POST /onboarding/complete to mark user onboarded and fire initial face ranking.