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/RecipeReferencePanel.svelte

@@ -30,7 +30,7 @@
 					href="/recipes/{slot.recipe?.id}"
 					class="flex items-center gap-2 rounded-[var(--radius-md)] border border-[var(--color-border)] bg-[var(--color-page)] px-3 py-2 hover:border-[var(--green-light)]"
 				>
-					<span class="min-w-[28px] font-[var(--font-sans)] text-[11px] text-[var(--color-text-muted)]">
+					<span class="min-w-[28px] font-[var(--font-sans)] text-[12px] text-[var(--color-text-muted)]">
 						{slot.slotDate ? formatDayAbbr(slot.slotDate, 'short') : ''}
 					</span>
 					<span class="flex-1 truncate font-[var(--font-sans)] text-[13px] font-medium text-[var(--color-text)]">