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/routes/geschichten/[id]/edit/+page.svelte

@@ -32,7 +32,7 @@ async function handleSubmit(payload: {
 		if (!res.ok) {
 			const code = (await res.json().catch(() => ({})))?.code;
 			errorMessage = getErrorMessage(code);
-			return;
+			throw new Error('save failed');
 		}
 		goto(`/geschichten/${data.geschichte.id}`);
 	} finally {