feat(shopping): add ChecklistItem.svelte component

Checkbox row with name, quantity/unit, recipe source label, and
strikethrough styling when checked. Each toggle submits a form action.

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

frontend/src/lib/shopping/ChecklistItem.svelte

@@ -0,0 +1,79 @@
+<script lang="ts">
+	import { enhance } from '$app/forms';
+
+	interface RecipeRef {
+		id?: string;
+		name?: string;
+	}
+
+	interface Props {
+		listId: string;
+		itemId: string;
+		name: string;
+		quantity: number | null;
+		unit: string | null;
+		isChecked: boolean;
+		sourceRecipes: RecipeRef[];
+	}
+
+	let { listId, itemId, name, quantity, unit, isChecked, sourceRecipes }: Props = $props();
+
+	let recipeLabel = $derived(
+		sourceRecipes.length > 0
+			? sourceRecipes
+					.map((r) => r.name)
+					.filter(Boolean)
+					.join(', ')
+			: null
+	);
+
+	let quantityLabel = $derived(
+		quantity ? `${quantity}${unit ? ` ${unit}` : ''}` : null
+	);
+</script>
+
+<form method="POST" action="?/check" use:enhance class="group flex items-center gap-3 py-2">
+	<input type="hidden" name="listId" value={listId} />
+	<input type="hidden" name="itemId" value={itemId} />
+	<input type="hidden" name="isChecked" value={!isChecked} />
+
+	<button
+		type="submit"
+		aria-label="{isChecked ? 'Abhaken rückgängig' : 'Abhaken'}: {name}"
+		class="flex h-5 w-5 flex-shrink-0 items-center justify-center rounded border
+			{isChecked
+			? 'border-[var(--green)] bg-[var(--green)] text-white'
+			: 'border-[var(--color-border)] bg-[var(--color-surface)] hover:border-[var(--green-light)]'}"
+	>
+		{#if isChecked}
+			<svg class="h-3 w-3" viewBox="0 0 12 12" fill="none" stroke="currentColor" stroke-width="2">
+				<path d="M2 6l3 3 5-5" />
+			</svg>
+		{/if}
+	</button>
+
+	<div class="min-w-0 flex-1">
+		<p
+			class="font-[var(--font-sans)] text-[14px] {isChecked
+				? 'text-[var(--color-text-muted)] line-through'
+				: 'text-[var(--color-text)]'}"
+		>
+			{name}
+		</p>
+		{#if recipeLabel && !isChecked}
+			<p class="truncate font-[var(--font-sans)] text-[11px] text-[var(--color-text-muted)]">
+				Für: {recipeLabel}
+			</p>
+		{/if}
+	</div>
+
+	{#if quantityLabel}
+		<span
+			class="flex-shrink-0 font-[var(--font-sans)] text-[13px] {isChecked
+				? 'text-[var(--color-text-muted)]'
+				: 'text-[var(--color-text)]'}"
+		>
+			{quantityLabel}
+		</span>
+	{/if}
+</form>