fix(geschichte-detail): span directory width with centered reading column

The detail page stayed at max-w-3xl and looked narrower than every
other page. The outer container now matches the directory width
(max-w-7xl) so the sheet spans the page like Dokumente/Personen, while
an inner max-w-3xl column keeps the prose line length readable.

Refs #799
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/specs/lesereisen-reader-spec.html

@@ -629,7 +629,7 @@
           <tbody>
             <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-Container</td><td>max-w-7xl mx-auto px-4 py-8; innere Lesespalte: max-w-3xl mx-auto</td><td>gleich wie StoryReader (R-2)</td></tr>
             <tr><td>Artikel-Sheet</td><td>rounded-sm border border-line bg-sheet shadow-sm px-5 py-6 sm:px-10 sm:py-10</td><td>Lesebogen-Panel zwischen Canvas und weißen Karten (Token --color-sheet), 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-journey-tint text-journey border border-journey-border 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>