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);
 		}
 	});

frontend/src/routes/layout.css

@@ -34,6 +34,9 @@
 	--color-overlay: var(--c-overlay);
 	--color-muted: var(--c-muted);
 
+	/* Reading sheet — article panel between canvas and the white cards it contains */
+	--color-sheet: var(--c-sheet);
+
 	/* Borders */
 	--color-line: var(--c-line);
 	--color-line-2: var(--c-line-2);
@@ -106,6 +109,7 @@
 	--c-surface: #ffffff;
 	--c-overlay: #ffffff;
 	--c-muted: #f5f4ef;
+	--c-sheet: #fafaf7; /* between canvas and surface — spec .g-article value */
 
 	--c-line: #e4e2d7;
 	--c-line-2: #eeede8;
@@ -213,6 +217,7 @@
 		--c-surface: #011526;
 		--c-overlay: #011e38;
 		--c-muted: #011a30;
+		--c-sheet: #011222; /* between canvas and surface */
 
 		--c-line: #0d3358;
 		--c-line-2: #092843;
@@ -306,6 +311,7 @@
 	--c-surface: #011526;
 	--c-overlay: #011e38;
 	--c-muted: #011a30;
+	--c-sheet: #011222; /* between canvas and surface */
 
 	--c-line: #0d3358;
 	--c-line-2: #092843;