feat(geschichte): wire StoryDocumentPanel into the story editor sidebar (#795)

GeschichteSidebar gains optional geschichteId/items props and renders the
panel only when geschichteId is set. GeschichteEditor derives both from
its existing GeschichteView prop — null on /geschichten/new, so the panel
stays hidden there (create-then-edit, same as journeys). JourneyEditor's
sidebar call site is untouched, so journeys never show the panel.

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

frontend/src/lib/geschichte/GeschichteSidebar.svelte.spec.ts

@@ -0,0 +1,40 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import { m } from '$lib/paraglide/messages.js';
+import GeschichteSidebar from './GeschichteSidebar.svelte';
+
+const item = {
+	id: 'i1',
+	position: 10,
+	document: {
+		id: 'd1',
+		title: 'Brief von Eugenie',
+		datePrecision: 'DAY' as const,
+		receiverCount: 0
+	}
+};
+
+afterEach(() => cleanup());
+
+describe('GeschichteSidebar — document panel contract (#795)', () => {
+	it('renders the document panel when geschichteId and items are provided', async () => {
+		render(GeschichteSidebar, {
+			status: 'DRAFT',
+			selectedPersons: [],
+			geschichteId: 'g1',
+			items: [item]
+		});
+
+		await expect
+			.element(page.getByRole('heading', { name: m.geschichte_documents_heading() }))
+			.toBeInTheDocument();
+		await expect.element(page.getByText('Brief von Eugenie')).toBeInTheDocument();
+	});
+
+	it('does not render the document panel without geschichteId', async () => {
+		render(GeschichteSidebar, { status: 'DRAFT', selectedPersons: [] });
+
+		expect(document.body.textContent).not.toContain(m.geschichte_documents_heading());
+	});
+});