fix(persistence): harden coerceEpoch null guard + autosave catch rollback

- coerceEpoch: early-return fallback on null/undefined input (was falling
  through to new Date(null) → epoch 0, surfacing as 1970-01-01 in the
  stories list); also unify the tail branch to Number.isFinite for
  consistency with the number-type fast path
- autosave .catch: roll back lastSavedFingerprintRef on unexpected throw
  so the next session change retries instead of silently marking the
  content as saved

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

app/[locale]/play/page.tsx

@@ -902,10 +902,11 @@ function PlayInner() {
           lastSavedFingerprintRef.current = "";
         }
       })
-      // Defensive: saveStory is contracted never to throw, but if a future edit
-      // to this callback ever does, an unhandled rejection here would poison the
-      // chain and freeze ALL subsequent saves. Swallow to keep the chain alive.
-      .catch(() => {});
+      .catch(() => {
+        if (lastSavedFingerprintRef.current === fingerprint) {
+          lastSavedFingerprintRef.current = "";
+        }
+      });
   }, [session]);
   useEffect(() => {
     currentSceneRef.current = currentScene;