fix(journey-editor): unsaved-warning banner + save throws on failure

JourneyEditor now renders UnsavedWarningBanner when showUnsavedWarning
is true. save() wraps onSubmit in try/catch so clearOnSuccess only fires
on success. edit/+page.svelte handleSubmit throws instead of returning
on non-ok responses so JourneyEditor sees the failure.

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

frontend/src/lib/geschichte/JourneyEditor.svelte

@@ -11,6 +11,7 @@ import type { DocumentOption } from '$lib/document/documentTypeahead';
 import GeschichteSidebar from './GeschichteSidebar.svelte';
 import JourneyItemRow from './JourneyItemRow.svelte';
 import JourneyAddBar from './JourneyAddBar.svelte';
+import UnsavedWarningBanner from '$lib/shared/primitives/UnsavedWarningBanner.svelte';
 
 type GeschichteView = components['schemas']['GeschichteView'];
 type JourneyItemView = components['schemas']['JourneyItemView'];
@@ -224,19 +225,27 @@ async function handleMoveDown(index: number) {
 async function save(nextStatus: 'DRAFT' | 'PUBLISHED') {
 	titleTouched = true;
 	if (titleEmpty) return;
-	await onSubmit({
-		title: title.trim(),
-		body,
-		status: nextStatus,
-		personIds: selectedPersons.map((p) => p.id!).filter(Boolean)
-	});
-	unsaved.clearOnSuccess();
+	try {
+		await onSubmit({
+			title: title.trim(),
+			body,
+			status: nextStatus,
+			personIds: selectedPersons.map((p) => p.id!).filter(Boolean)
+		});
+		unsaved.clearOnSuccess();
+	} catch {
+		// onSubmit signalled failure — keep dirty flag so the banner stays
+	}
 }
 </script>
 
 <!-- Screen-reader live region for move announcements -->
 <div aria-live="polite" aria-atomic="true" class="sr-only">{liveAnnounce}</div>
 
+{#if unsaved.showUnsavedWarning}
+	<UnsavedWarningBanner onDiscard={unsaved.discard} />
+{/if}
+
 <div class="grid grid-cols-1 gap-6 lg:grid-cols-[2fr_1fr]">
 	<!-- Editor column -->
 	<div bind:this={editorColEl} class="flex flex-col gap-4">