fix(richtlinien): <main> landmark + closing card h2 → h3

- Wrap page content in <main> so AT users can jump to main content (Nora)
- Closing card "Fehlt eine Regel?" was <h2> after two existing <h2> siblings
  but styled like a card title, not a section label; downgrade to <h3> to
  fix the heading hierarchy (Sara/Leonie)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/hilfe/transkription/+page.svelte

@@ -46,7 +46,7 @@ const klaerungChips = [
 	<title>{m.richtlinien_title()} — Familienarchiv</title>
 </svelte:head>
 
-<div class="mx-auto max-w-2xl px-4 py-10 font-serif">
+<main class="mx-auto max-w-2xl px-4 py-10 font-serif">
 	<!-- Title -->
 	<h1 class="mb-4 text-3xl font-bold text-ink">{m.richtlinien_title()}</h1>
 
@@ -102,10 +102,10 @@ const klaerungChips = [
 
 	<!-- Closing card -->
 	<div class="border-brand-sand rounded-sm border bg-white p-6 shadow-sm">
-		<h2 class="mb-2 font-serif text-lg font-bold text-ink">{m.richtlinien_closing_title()}</h2>
+		<h3 class="mb-2 font-serif text-lg font-bold text-ink">{m.richtlinien_closing_title()}</h3>
 		<p class="font-serif text-sm leading-relaxed text-ink-2">{m.richtlinien_closing_body()}</p>
 	</div>
-</div>
+</main>
 
 <style>
 @media print {