fix(journey-editor): render empty-state message when items list is empty

Previously the item list area was blank when no items had been added.
The empty-state paragraph uses the existing journey_empty_state i18n key.

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

frontend/src/lib/geschichte/JourneyEditor.svelte

@@ -244,6 +244,9 @@ async function save(nextStatus: 'DRAFT' | 'PUBLISHED') {
 			onpointerup={() => dragDrop.handlePointerUp()}
 			class="flex flex-col gap-2"
 		>
+			{#if items.length === 0}
+				<p class="font-sans text-sm text-ink-3">{m.journey_empty_state()}</p>
+			{/if}
 			{#each items as item, i (item.id)}
 				<!-- svelte-ignore a11y_no_static_element_interactions -->
 				<div