fix(shopping): address frontend review concerns

- ChecklistItem: use:enhance with reset:false, role=checkbox, aria-checked, focus ring
- RecipeReferencePanel: day abbreviation text-[12px] (was 11px)
- ShoppingHeader: generating pending state disables button during submit
- AddCustomItem: only collapse form on successful submission

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

frontend/src/lib/shopping/ChecklistItem.svelte

@@ -32,15 +32,17 @@
 	);
 </script>
 
-<form method="POST" action="?/check" use:enhance class="group flex items-center gap-3 py-2">
+<form method="POST" action="?/check" use:enhance={() => async ({ update }) => update({ reset: false })} 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"
+		role="checkbox"
+		aria-checked={isChecked}
 		aria-label="{isChecked ? 'Abhaken rückgängig' : 'Abhaken'}: {name}"
-		class="flex h-5 w-5 flex-shrink-0 items-center justify-center rounded border
+		class="flex h-5 w-5 flex-shrink-0 items-center justify-center rounded border focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-[var(--green)]
 			{isChecked
 			? 'border-[var(--green)] bg-[var(--green)] text-white'
 			: 'border-[var(--color-border)] bg-[var(--color-surface)] hover:border-[var(--green-light)]'}"