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/DocumentPickerDropdown.svelte.spec.ts

@@ -95,6 +95,89 @@ describe('DocumentPickerDropdown — selection', () => {
 	});
 });
 
+describe('DocumentPickerDropdown — keyboard navigation', () => {
+	it('selects the first option via ArrowDown then Enter', async () => {
+		const onSelect = vi.fn();
+		mockSearchResponse([docFactory('d1', 'Brief von Eugenie'), docFactory('d2', 'Brief 2')]);
+
+		render(DocumentPickerDropdown, { onSelect });
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+		await userEvent.keyboard('{ArrowDown}');
+		await userEvent.keyboard('{Enter}');
+
+		expect(onSelect).toHaveBeenCalledWith(expect.objectContaining({ id: 'd1' }));
+	});
+
+	it('does not select an aria-disabled option on Enter', 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.keyboard('{ArrowDown}');
+		await userEvent.keyboard('{Enter}');
+
+		expect(onSelect).not.toHaveBeenCalled();
+	});
+
+	it('closes the dropdown on Escape', async () => {
+		mockSearchResponse([docFactory('d1', 'Brief von Eugenie')]);
+
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+		await expect.element(page.getByRole('listbox')).toBeInTheDocument();
+
+		await userEvent.keyboard('{Escape}');
+
+		await expect.element(page.getByRole('listbox')).not.toBeInTheDocument();
+	});
+
+	it('points aria-activedescendant at the active option', async () => {
+		mockSearchResponse([docFactory('d1', 'Brief von Eugenie'), docFactory('d2', 'Brief 2')]);
+
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		const input = page.getByRole('combobox');
+		await userEvent.fill(input, 'Brief');
+		await waitForDebounce();
+
+		expect(input.element().getAttribute('aria-activedescendant')).toBeNull();
+
+		await userEvent.keyboard('{ArrowDown}');
+
+		const activeId = input.element().getAttribute('aria-activedescendant');
+		expect(activeId).toMatch(/-option-0$/);
+		const firstOption = page
+			.getByText(/Brief von Eugenie/i)
+			.element()
+			.closest('li')!;
+		expect(firstOption.id).toBe(activeId);
+		expect(firstOption.getAttribute('aria-selected')).toBe('true');
+	});
+});
+
+describe('DocumentPickerDropdown — no results', () => {
+	it('shows a non-interactive no-results row when the search returns zero hits', async () => {
+		mockSearchResponse([]);
+
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		await userEvent.fill(page.getByRole('combobox'), 'xyz');
+		await waitForDebounce();
+
+		await expect.element(page.getByText(m.comp_typeahead_no_results())).toBeInTheDocument();
+	});
+});
+
 describe('DocumentPickerDropdown — search failure', () => {
 	it('shows an error message when the search request fails instead of vanishing', async () => {
 		// 500 from /api/documents/search — must surface, not render as "no results"