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/geschichten-reader-journey-spec.html

@@ -641,7 +641,7 @@
           <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-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>Article 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); 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>