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/shared/hooks/useTypeahead.svelte.test.ts

@@ -106,6 +106,17 @@ describe('createTypeahead', () => {
 		errorSpy.mockRestore();
 	});
 
+	it('sets loading immediately on setQuery so empty results read as pending, not "no results"', async () => {
+		const fetchUrl = vi.fn().mockResolvedValue([]);
+		const ta = createTypeahead({ fetchUrl, debounceMs: 300 });
+		ta.setQuery('foo');
+		// During the debounce window no fetch has run yet — callers must be able to
+		// distinguish "still searching" from "searched, zero hits".
+		expect(ta.loading).toBe(true);
+		await vi.advanceTimersByTimeAsync(300);
+		expect(ta.loading).toBe(false);
+	});
+
 	it('setActiveIndex updates activeIndex', () => {
 		const ta = createTypeahead({ fetchUrl: vi.fn().mockResolvedValue([]) });
 		expect(ta.activeIndex).toBe(-1);

frontend/src/lib/shared/hooks/useTypeahead.svelte.ts

@@ -19,9 +19,11 @@ export function createTypeahead<T>(options: Options<T>) {
 	function setQuery(q: string) {
 		query = q;
 		isOpen = true;
+		// Set loading before the debounce fires so callers can distinguish
+		// "still searching" from "searched, zero hits" during the debounce window.
+		loading = true;
 		clearTimeout(debounceTimer);
 		debounceTimer = setTimeout(async () => {
-			loading = true;
 			error = false;
 			try {
 				results = await fetchUrl(q);