feat(recipes): display form error from \$page.form in RecipeForm

- Import page store and render role="alert" error banner
- Add mock for \$app/stores and \$app/forms in RecipeForm tests
- Add tests: error banner shown when form.error set, hidden when null

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/recipes/RecipeForm.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+	import { page } from '$app/stores';
+
 	type Category = { id: string; name: string; tagType?: string };
 
 	type EditRecipe = {
@@ -45,6 +47,11 @@
 </script>
 
 <form method="POST" {action}>
+	<!-- Error banner -->
+	{#if $page.form?.error}
+		<div role="alert">{$page.form.error}</div>
+	{/if}
+
 	<!-- Basic info -->
 	<label for="name">Name</label>
 	<input id="name" name="name" type="text" bind:value={name} required />