fix(typeahead): surface search failures instead of an empty dropdown

The document picker parsed the response without checking r.ok, so a 401/500
rendered identically to 'no matches' and the dropdown silently vanished —
which is how the broken relevance path shipped invisibly. The fetch now
throws on non-OK, the useTypeahead hook exposes an error flag, and the
picker renders a visible failure message (de/en/es).

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

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

@@ -78,6 +78,34 @@ describe('createTypeahead', () => {
 		errorSpy.mockRestore();
 	});
 
+	it('fetch error sets the error flag so callers can render a failure state', async () => {
+		const fetchUrl = vi.fn().mockRejectedValue(new Error('500'));
+		const errorSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+		const ta = createTypeahead({ fetchUrl, debounceMs: 0 });
+		expect(ta.error).toBe(false);
+		ta.setQuery('foo');
+		await vi.advanceTimersByTimeAsync(0);
+		expect(ta.error).toBe(true);
+		errorSpy.mockRestore();
+	});
+
+	it('error flag clears on the next successful fetch', async () => {
+		const fetchUrl = vi
+			.fn()
+			.mockRejectedValueOnce(new Error('500'))
+			.mockResolvedValueOnce([{ id: '1' }]);
+		const errorSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+		const ta = createTypeahead({ fetchUrl, debounceMs: 0 });
+		ta.setQuery('foo');
+		await vi.advanceTimersByTimeAsync(0);
+		expect(ta.error).toBe(true);
+		ta.setQuery('foob');
+		await vi.advanceTimersByTimeAsync(0);
+		expect(ta.error).toBe(false);
+		expect(ta.results).toEqual([{ id: '1' }]);
+		errorSpy.mockRestore();
+	});
+
 	it('setActiveIndex updates activeIndex', () => {
 		const ta = createTypeahead({ fetchUrl: vi.fn().mockResolvedValue([]) });
 		expect(ta.activeIndex).toBe(-1);

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

@@ -11,6 +11,7 @@ export function createTypeahead<T>(options: Options<T>) {
 	let results: T[] = $state([]);
 	let isOpen = $state(false);
 	let loading = $state(false);
+	let error = $state(false);
 	let activeIndex = $state(-1);
 
 	let debounceTimer: ReturnType<typeof setTimeout> | undefined;
@@ -21,11 +22,13 @@ export function createTypeahead<T>(options: Options<T>) {
 		clearTimeout(debounceTimer);
 		debounceTimer = setTimeout(async () => {
 			loading = true;
+			error = false;
 			try {
 				results = await fetchUrl(q);
 			} catch (e) {
 				console.error('typeahead fetch error', e);
 				results = [];
+				error = true;
 			} finally {
 				loading = false;
 			}
@@ -65,6 +68,9 @@ export function createTypeahead<T>(options: Options<T>) {
 		get loading() {
 			return loading;
 		},
+		get error() {
+			return error;
+		},
 		get activeIndex() {
 			return activeIndex;
 		},