feat: add create document feature via web interface

- Backend: new POST /api/documents endpoint with DocumentService.createDocument()
  reusing DocumentUpdateDTO; handles file upload, tags, sender, receivers
- Frontend: new /documents/new route with same four-section form as edit page
  (Wer & Wann, Beschreibung, Transkription, Datei) but with empty fields
- Home page: subtle '+ Neues Dokument' link above the document list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/new/+page.server.ts

@@ -0,0 +1,35 @@
+import { fail, redirect } from '@sveltejs/kit';
+import { env } from '$env/dynamic/private';
+import { createApiClient } from '$lib/api.server';
+import { parseBackendError, getErrorMessage } from '$lib/errors';
+
+export async function load({ fetch }) {
+	const api = createApiClient(fetch);
+	const personsResult = await api.GET('/api/persons');
+
+	if (!personsResult.response.ok) {
+		return { persons: [] };
+	}
+
+	return { persons: personsResult.data };
+}
+
+export const actions = {
+	default: async ({ request, fetch }) => {
+		const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+		const formData = await request.formData();
+
+		const res = await fetch(`${baseUrl}/api/documents`, {
+			method: 'POST',
+			body: formData
+		});
+
+		if (!res.ok) {
+			const backendError = await parseBackendError(res);
+			return fail(res.status, { error: getErrorMessage(backendError?.code) });
+		}
+
+		const created = await res.json();
+		throw redirect(303, `/documents/${created.id}`);
+	}
+};