Add the setup flow's welcome screen and shell

The front door reads the same ledger every screen reads: a record that
never entered the flow sees the welcome, one mid-way is sent to the step
it owes, a finished one is offered the questions again. The shell prints
the counter and an ordered step list off the step machine, so it shows
exactly the screens this flow has, and every screen moves focus to its
heading on arrival.

"Skip for now" is a real exit — it stamps the account past the first-run
redirect and opens the checklist on the dashboard — and the one-time
medical disclaimer now gates it as well as "Set up", because a skip that
reached the dashboard without it would be a way round the acknowledgment.

The five-step wizard's welcome carousel, goal picker and source grid go
with it; the numbered step route follows in the next commit.

src/__tests__/success-affordance-guard.test.ts

@@ -275,7 +275,6 @@ const PINNED_AFFORDANCES: Record<
   "src/components/mood/mood-form.tsx": { "toast.success": 1 },
   "src/components/mood/mood-list.tsx": { "toast.success": 3 },
   "src/components/onboarding/done-screen.tsx": { CheckCircle2: 2 },
-  "src/components/onboarding/source-card-grid.tsx": { CheckCircle2: 2 },
   "src/components/records/allergy-form.tsx": { "toast.success": 1 },
   "src/components/records/allergy-manager.tsx": { "toast.success": 1 },
   "src/components/records/conditions-manager.tsx": { "toast.success": 1 },