feat(geschichte-detail): render article on a reading-sheet surface card

The R-2 mockup shows the article on a distinct light panel, but the
impl-ref table only specified the centered container, so the page
rendered flat on the canvas. Wrap the <article> in the standard card
pattern (BackButton stays outside on the canvas) and record the sheet
in both spec impl-ref tables so mockup and impl-ref agree.

Closes #797
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/specs/lesereisen-reader-spec.html

@@ -630,6 +630,7 @@
             <tr class="grp"><td colspan="3">Seitenstruktur</td></tr>
             <tr><td>Bedingte Logik</td><td>{#if geschichte.type === 'JOURNEY'} JourneyReader {:else} StoryReader {/if}</td><td>in +page.svelte von /geschichten/[id]</td></tr>
             <tr><td>Artikel-Container</td><td>max-w-3xl mx-auto px-4 py-8</td><td>gleich wie StoryReader</td></tr>
+            <tr><td>Artikel-Sheet</td><td>rounded-sm border border-line bg-surface shadow-sm px-5 py-6 sm:px-10 sm:py-10</td><td>Lesebogen-Panel auf dem Canvas, gleich wie Story (R-2); BackButton bleibt außerhalb</td></tr>
             <tr><td>Journey-Badge</td><td>inline-flex px-2 py-px rounded-sm text-[10px] font-bold uppercase tracking-widest bg-orange-50 text-orange-700 border border-orange-200 mb-2</td><td>über dem Titel; nicht für STORY</td></tr>
             <tr><td>Titel</td><td>font-serif text-3xl text-ink leading-tight mb-4</td><td>gleich wie Story</td></tr>
             <tr><td>Metabar</td><td>flex items-center gap-3 pb-4 border-b border-subtle mb-4</td><td>gleich wie Story</td></tr>