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>

frontend/src/routes/geschichten/[id]/+page.svelte

@@ -55,7 +55,7 @@ async function handleDelete() {
 
 	<article
 		aria-labelledby="geschichte-title"
-		class="rounded-sm border border-line bg-surface px-5 py-6 shadow-sm sm:px-10 sm:py-10"
+		class="rounded-sm border border-line bg-sheet px-5 py-6 shadow-sm sm:px-10 sm:py-10"
 	>
 		<header class="mb-6">
 			{#if isJourney}

frontend/src/routes/geschichten/[id]/page.svelte.test.ts

@@ -78,7 +78,8 @@ describe('geschichten/[id] page', () => {
 
 		const article = document.querySelector('article');
 		expect(article).not.toBeNull();
-		for (const cls of ['bg-surface', 'border-line', 'rounded-sm', 'shadow-sm']) {
+		// bg-sheet sits between the sand canvas and the white cards inside the article
+		for (const cls of ['bg-sheet', 'border-line', 'rounded-sm', 'shadow-sm']) {
 			expect(article!.className).toContain(cls);
 		}
 	});