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/routes/geschichten/new/StoryCreate.svelte.spec.ts

@@ -0,0 +1,16 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { m } from '$lib/paraglide/messages.js';
+
+vi.mock('$app/navigation', () => ({ beforeNavigate: vi.fn(), goto: vi.fn() }));
+import StoryCreate from './StoryCreate.svelte';
+
+afterEach(() => cleanup());
+
+describe('StoryCreate — document panel guard (#795)', () => {
+	it('renders without the document panel — documents attach after the first save', async () => {
+		render(StoryCreate, { initialPersons: [] });
+
+		expect(document.body.textContent).not.toContain(m.geschichte_documents_heading());
+	});
+});