feat(journey-editor): build DocumentPickerDropdown + refactor DocumentMultiSelect

New DocumentPickerDropdown: single-select document search with aria-disabled
for already-added items and sr-only "bereits enthalten" hint. DocumentMultiSelect
refactored to use createTypeahead, removing raw setTimeout/debounceTimer.

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

frontend/src/lib/document/DocumentPickerDropdown.svelte.spec.ts

@@ -0,0 +1,92 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page, userEvent } from 'vitest/browser';
+import DocumentPickerDropdown from './DocumentPickerDropdown.svelte';
+
+const waitForDebounce = () => new Promise((r) => setTimeout(r, 350));
+
+const docFactory = (id: string, title: string) => ({
+	id,
+	title,
+	documentDate: '1880-01-01',
+	metaDatePrecision: 'DAY' as const,
+	metaDateEnd: undefined
+});
+
+function mockSearchResponse(items: ReturnType<typeof docFactory>[]) {
+	vi.stubGlobal(
+		'fetch',
+		vi.fn().mockResolvedValue({
+			ok: true,
+			json: vi.fn().mockResolvedValue({ items })
+		})
+	);
+}
+
+afterEach(() => {
+	cleanup();
+	vi.unstubAllGlobals();
+});
+
+describe('DocumentPickerDropdown — empty query guard', () => {
+	it('does not call fetch on empty query', async () => {
+		const fetchMock = vi.fn();
+		vi.stubGlobal('fetch', fetchMock);
+
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+		await userEvent.fill(page.getByRole('combobox'), '');
+		await waitForDebounce();
+
+		expect(fetchMock).not.toHaveBeenCalled();
+	});
+});
+
+describe('DocumentPickerDropdown — already-added indicator', () => {
+	it('shows already-added document as aria-disabled with sr-only hint', async () => {
+		mockSearchResponse([docFactory('d1', 'Brief von Eugenie'), docFactory('d2', 'Brief 2')]);
+
+		render(DocumentPickerDropdown, {
+			alreadyAddedIds: new Set(['d1']),
+			onSelect: vi.fn()
+		});
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+
+		const disabledOption = page.getByRole('option', { name: /Brief von Eugenie/i });
+		await expect.element(disabledOption).toHaveAttribute('aria-disabled', 'true');
+		// Screen-reader text "bereits enthalten" must be present in the option
+		await expect.element(page.getByText(/bereits enthalten/i)).toBeInTheDocument();
+	});
+});
+
+describe('DocumentPickerDropdown — selection', () => {
+	it('calls onSelect with the item when a non-disabled option is clicked', async () => {
+		const onSelect = vi.fn();
+		mockSearchResponse([docFactory('d1', 'Brief von Eugenie')]);
+
+		render(DocumentPickerDropdown, { onSelect });
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+		await userEvent.click(page.getByRole('option', { name: /Brief von Eugenie/i }));
+
+		expect(onSelect).toHaveBeenCalledWith(expect.objectContaining({ id: 'd1' }));
+	});
+
+	it('does not call onSelect when an aria-disabled option is clicked', async () => {
+		const onSelect = vi.fn();
+		mockSearchResponse([docFactory('d1', 'Brief von Eugenie')]);
+
+		render(DocumentPickerDropdown, {
+			alreadyAddedIds: new Set(['d1']),
+			onSelect
+		});
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+		await userEvent.click(page.getByRole('option', { name: /Brief von Eugenie/i }));
+
+		expect(onSelect).not.toHaveBeenCalled();
+	});
+});