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/geschichten-reader-journey-spec.html

@@ -640,7 +640,8 @@
           <thead><tr><th>Element</th><th>Wert</th><th>Hinweise</th></tr></thead>
           <tbody>
             <tr class="grp"><td colspan="3">Artikel-Container</td></tr>
-            <tr><td>Article container</td><td>max-w-3xl mx-auto px-4 py-10</td><td>zentriert, volle Breite auf Mobile</td></tr>
+            <tr><td>Article container</td><td>max-w-3xl mx-auto px-4 py-8</td><td>zentriert, volle Breite auf Mobile</td></tr>
+            <tr><td>Article 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 (siehe Mockup); BackButton bleibt außerhalb</td></tr>
             <tr><td>Story title</td><td>font-family:var(--font-display);font-size:clamp(22px,4vw,32px);color:var(--navy)</td><td>Fraunces, nicht fett</td></tr>
             <tr><td>Back button</td><td>&lt;BackButton /&gt; aus $lib/components/BackButton.svelte</td><td>history.back(); nicht &lt;a href&gt;</td></tr>
             <tr class="grp"><td colspan="3">Metazeile</td></tr>