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/document/DocumentPickerDropdown.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 DocumentPickerDropdown from './DocumentPickerDropdown.svelte';
+import { m } from '$lib/paraglide/messages.js';
 
 const waitForDebounce = () => new Promise((r) => setTimeout(r, 350));
 
@@ -93,3 +94,24 @@ describe('DocumentPickerDropdown — selection', () => {
 		expect(onSelect).not.toHaveBeenCalled();
 	});
 });
+
+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"
+		vi.stubGlobal(
+			'fetch',
+			vi.fn().mockResolvedValue({
+				ok: false,
+				status: 500,
+				json: vi.fn().mockResolvedValue({ code: 'INTERNAL_ERROR' })
+			})
+		);
+
+		render(DocumentPickerDropdown, { onSelect: vi.fn() });
+
+		await userEvent.fill(page.getByRole('combobox'), 'Brief');
+		await waitForDebounce();
+
+		await expect.element(page.getByText(m.comp_typeahead_error())).toBeInTheDocument();
+	});
+});