fix(geschichte): uniform onSubmit rejects-on-failure contract

The c3afd57e fix made the edit page's handleSubmit throw on !res.ok but
only JourneyEditor caught it. Now: GeschichteEditor.save() catches and
keeps its dirty state (no unhandled rejection -> no GlitchTip noise on a
failed STORY save); StoryCreate throws on failure so a failed STORY
create no longer silently disarms the unsaved guard; both handleSubmit
implementations catch network rejections, surface a message, and rethrow.
Contract documented on both editors' Props. GeschichteEditor also gets
the title maxlength=255. Spec: rejecting onSubmit is caught and the
editor stays usable.

Review round 3: Felix §2, Tobias S1, Nora (3), Markus (concern 1).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/routes/geschichten/new/StoryCreate.svelte

@@ -31,10 +31,18 @@ async function handleSubmit(payload: {
 		if (!res.ok) {
 			const code = (await res.json().catch(() => ({})))?.code;
 			errorMessage = getErrorMessage(code);
-			return;
+			throw new Error('create failed');
 		}
 		const created = await res.json();
 		goto(`/geschichten/${created.id}`);
+	} catch (e) {
+		if (!errorMessage) {
+			console.error('Story create failed', e);
+			errorMessage = getErrorMessage(undefined);
+		}
+		// Contract: onSubmit rejects on failure — GeschichteEditor catches and
+		// keeps its dirty state instead of disarming the unsaved-changes guard.
+		throw e;
 	} finally {
 		submitting = false;
 	}