feat(ui): add sheet surface token between canvas and white cards

The reading sheet used bg-surface (white), so the document cards inside
the article had the same background as the sheet itself. The spec's
three-level hierarchy is canvas → article panel (#FAFAF7) → white cards;
introduce --color-sheet (mode-aware) and use it on the article. Also
move JourneyItemCard from bg-white to bg-surface so dark mode remaps it,
and tint the curator annotation with bg-muted so it stands off the card.

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

docs/specs/lesereisen-reader-spec.html

@@ -630,7 +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>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-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>
@@ -639,12 +639,12 @@
             <tr><td>Intro (body)</td><td>font-serif text-sm text-ink-2 italic leading-relaxed mb-6 pb-4 border-b border-dashed border-subtle</td><td>nur rendern wenn body nicht leer; kein HTML-Rendering — plaintext</td></tr>
             <tr class="grp"><td colspan="3">Dokument-Item</td></tr>
             <tr><td>Item-Zeile</td><td>mb-3</td><td>kein flex nötig — Karte ist full-width</td></tr>
-            <tr><td>Dokumentkarte</td><td>bg-white border border-line rounded-sm p-3</td><td></td></tr>
+            <tr><td>Dokumentkarte</td><td>bg-surface border border-line rounded-sm p-3</td><td></td></tr>
             <tr><td>Brieftitel</td><td>font-serif text-sm text-ink leading-snug mb-0.5</td><td>document.title</td></tr>
             <tr><td>Briefmeta</td><td>text-xs text-ink-3 mb-2</td><td>formatDate(document.documentDate) · "von X an Y"</td></tr>
             <tr><td>Brief öffnen Link</td><td>inline-flex items-center gap-1 text-xs font-semibold text-ink hover:text-primary</td><td>href="/documents/{item.document.id}"</td></tr>
             <tr class="grp"><td colspan="3">Kuratoren-Annotation</td></tr>
-            <tr><td>Annotation</td><td>mt-3 pl-3 border-l-2 border-mint bg-surface rounded-r-sm py-1.5 pr-2</td><td>nur rendern wenn item.note vorhanden</td></tr>
+            <tr><td>Annotation</td><td>mt-3 pl-3 border-l-2 border-brand-mint bg-muted rounded-r-sm py-1.5 pr-2</td><td>nur rendern wenn item.note vorhanden</td></tr>
             <tr><td>Annotations-Text</td><td>text-xs italic text-ink-2 leading-relaxed</td><td></td></tr>
             <tr class="grp"><td colspan="3">Interlude-Item</td></tr>
             <tr><td>Interlude-Block</td><td>pl-3 border-l-2 border-orange-400 bg-orange-50 rounded-r-sm py-2 pr-3 my-4</td><td>item.document === null</td></tr>