feat: the homepage leads the visitor through as one story

The page was a row of sections to scroll past, and a film in one of them did
not change that. It now follows the questions a visitor has, in the order
they come up: who is this, can he do what I need, what does it look like,
what else, how does it go, has he done it, what is open, how do I start.

Each chapter ends by asking the next question as a large link that leads on,
and the next chapter answers it. A rail at the edge shows every chapter as a
station with a dot travelling along it (on phones: a pill with the current
chapter and one tap to the next), and the header names the current question.

Where the visitor is, is pure logic in src/domain/chapters.ts. The scroll
film section and the bridge sentences are removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/components/home/PortraitSection.tsx

@@ -22,6 +22,7 @@ export const PortraitSection: React.FC<{
 }> = ({ id, index, label, title, portraitAlt, children }) => (
   <section
     id={id}
+    data-chapter={id}
     className="relative scroll-mt-24 overflow-hidden bg-slate-900 text-white"
   >
     {/* Portrait: right side on wide screens, top on small ones. */}