Block create-list flow when no stores exist

Tapping "+ New list" with zero stores now shows a warning toast with an
"Add a store" action linking to /stores, instead of opening a form whose
required store dropdown would be empty.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/frontend/src/routes/lists/+page.svelte

@@ -1,6 +1,8 @@
 <script lang="ts">
 	import { onMount, onDestroy } from 'svelte';
+	import { goto } from '$app/navigation';
 	import { api } from '$lib/api';
+	import { toast } from '$lib/toast.svelte';
 	import { startConnection, stopConnection } from '$lib/signalr';
 	import type { HubConnection } from '@microsoft/signalr';
 
@@ -64,6 +66,16 @@
 		await stopConnection();
 	});
 
+	function toggleCreate() {
+		if (!showCreate && stores.length === 0) {
+			toast.warning('You need to create a store first', {
+				action: { label: 'Add a store', onClick: () => goto('/stores') }
+			});
+			return;
+		}
+		showCreate = !showCreate;
+	}
+
 	async function createList() {
 		if (!newName.trim() || !newStoreId) return;
 		await api<{ id: number }>('/api/lists', {
@@ -79,7 +91,7 @@
 	<div class="mb-4 flex items-center justify-between">
 		<h2 class="text-2xl font-bold">Shopping Lists</h2>
 		<button
-			onclick={() => (showCreate = !showCreate)}
+			onclick={toggleCreate}
 			class="rounded-full bg-primary px-4 py-2 text-sm font-semibold text-white"
 		>
 			{showCreate ? 'Cancel' : '+ New list'}