fix(share): harden story share and relocate import button

- Add Content-Length pre-check to story-pack and story-unpack routes
  to reject oversized payloads before buffering the body
- Suppress internal error details in story-unpack catch (was leaking
  e.message to the client)
- Strengthen sceneIndex validation: require non-negative integer
- Guard against undefined storyState when replaying shared stories
- Fix prefetch regression: remove currentBeat?.id from useEffect deps
  that was re-triggering all change-scene prefetches on every beat
- Fix double detach: use else-if so the second replay detach guard
  doesn't fire redundantly after the first already detached
- Align client file-size limit by format (.json 12MB, .infiplot 13MB)
- Move "载入剧情" import button next to "开始" with hover tooltip

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

lib/storyShare.ts

@@ -155,7 +155,11 @@ export function parseStoryShareDoc(value: unknown): StoryShareDoc {
   if (typeof value.exportedAt !== "number" || !Number.isFinite(value.exportedAt)) {
     throw new Error("剧情分享文件缺少导出时间");
   }
-  if (!isRecord(value.current) || typeof value.current.sceneIndex !== "number") {
+  if (
+    !isRecord(value.current) ||
+    !Number.isInteger(value.current.sceneIndex) ||
+    (value.current.sceneIndex as number) < 0
+  ) {
     throw new Error("剧情分享文件缺少当前位置");
   }
   if (