Add the setup flow's done screen

The dashboard is one click away, already ordered, and the checklist
opens on arrival. The tour is an offer here rather than an automatic
launch: pressing it sets the referrer the shell's launcher reads, and
nothing else does. Two record-shaped exits follow the answer to Q1: an
account that said "both" is offered the profile for the person it looks
after, and one that set up a record for somebody else is offered that
record through the same switch the account menu uses, so the first
entry lands where it belongs. The step is marked done on arrival, once.

src/app/onboarding/[step]/page.tsx

@@ -3,6 +3,7 @@ import { notFound, redirect } from "next/navigation";
 
 import { OnboardingShell } from "@/components/onboarding/onboarding-shell";
 import { ConfirmScreen } from "@/components/onboarding/confirm-screen";
+import { DoneScreen } from "@/components/onboarding/done-screen";
 import { FirstResultScreen } from "@/components/onboarding/first-result-screen";
 import { QuestionScreen } from "@/components/onboarding/question-screen";
 import { UnitsScreen } from "@/components/onboarding/units-screen";
@@ -106,5 +107,5 @@ function renderScreen(screen: OnboardingStepId, state: OnboardingStateDto) {
   if (screen === "first-result") {
     return <FirstResultScreen state={state} />;
   }
-  notFound();
+  return <DoneScreen state={state} />;
 }