frontend: re-home onboarding into app layout/spacing/type

[AndresL230]

[P1-C] components/screens/Onboarding.tsx is color-correct but structurally orphaned (centered card in a radial void, hard-coded px, bespoke type scale). Adopt --pad-* + the shell type scale; render inside the app's left-aligned frame. Deferred from token-unification Phase 1 (PR #286). Tracker: docs/superpowers/followups/2026-06-30-token-unification-followups.md · Audit: docs/frontend-rhythm-audit.md