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

@@ -62,13 +62,15 @@ function handleSelect(doc: DocumentOption) {
 		class="block w-full rounded border border-line bg-surface px-3 py-2 text-sm text-ink placeholder:text-ink-3 focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 	/>
 
-	{#if picker.isOpen && (picker.results.length > 0 || picker.loading)}
+	{#if picker.isOpen && (picker.results.length > 0 || picker.loading || picker.error)}
 		<ul
 			id={listboxId}
 			class="ring-opacity-5 absolute z-50 mt-1 max-h-60 w-full overflow-auto rounded-md bg-surface py-1 text-sm shadow-lg ring-1 ring-black"
 		>
 			{#if picker.loading}
 				<li class="px-3 py-2 text-ink-2">{m.comp_multiselect_loading()}</li>
+			{:else if picker.error}
+				<li role="alert" class="px-3 py-2 text-danger">{m.comp_typeahead_error()}</li>
 			{:else}
 				{#each picker.results as doc (doc.id)}
 					{@const disabled = alreadyAddedIds.has(doc.id!)}