fix(picker): honest combobox — keyboard navigation, listbox roles, no-results state

The input declared role=combobox + aria-autocomplete=list while arrow keys
did nothing (WCAG 4.1.2). Wired the useTypeahead activeIndex the same way
PersonTypeahead does: ArrowUp/Down cycle, Enter/Space select, Escape closes,
aria-activedescendant tracks the active option; the list is a real listbox
with option roles again (the interim role downgrade is reverted). Zero hits
now render a 'Keine Treffer' row instead of silently vanishing, aria-expanded
matches the visible state, and the hook sets loading at setQuery so the
debounce window can't read as 'no results'. DocumentMultiSelect renders the
shared error state too. _uid counter replaced with $props.id().

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

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

@@ -2,6 +2,7 @@ import { afterEach, describe, expect, it, vi } from 'vitest';
 import { cleanup, render } from 'vitest-browser-svelte';
 import { page, userEvent } from 'vitest/browser';
 import DocumentMultiSelect from './DocumentMultiSelect.svelte';
+import { m } from '$lib/paraglide/messages.js';
 
 const waitForDebounce = () => new Promise((r) => setTimeout(r, 350));
 
@@ -124,6 +125,28 @@ describe('DocumentMultiSelect — search and select', () => {
 	});
 });
 
+describe('DocumentMultiSelect — search failure', () => {
+	it('shows an error row when the search request fails instead of looking like "no results"', async () => {
+		vi.stubGlobal(
+			'fetch',
+			vi.fn().mockResolvedValue({
+				ok: false,
+				status: 500,
+				json: vi.fn().mockResolvedValue({ code: 'INTERNAL_ERROR' })
+			})
+		);
+
+		render(DocumentMultiSelect);
+
+		await userEvent.fill(page.getByPlaceholder('Dokument suchen…'), 'Eug');
+		await waitForDebounce();
+
+		const alert = page.getByRole('alert');
+		await expect.element(alert).toBeInTheDocument();
+		await expect.element(alert).toHaveTextContent(m.comp_typeahead_error());
+	});
+});
+
 describe('DocumentMultiSelect — remove', () => {
 	it('removes a chip when its × button is clicked', async () => {
 		render(DocumentMultiSelect, {