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>

app/api/story-pack/route.ts

@@ -13,6 +13,14 @@ export async function POST(req: Request): Promise<Response> {
     );
   }
 
+  const contentLength = req.headers.get("content-length");
+  if (contentLength && Number(contentLength) > MAX_DOC_BYTES + 1024) {
+    return Response.json(
+      { error: "剧情数据太大,无法打包分享" },
+      { status: 413 },
+    );
+  }
+
   let docStr: string;
   try {
     const body = (await req.json()) as { docStr?: unknown };

app/api/story-unpack/route.ts

@@ -13,6 +13,11 @@ export async function POST(req: Request): Promise<Response> {
     );
   }
 
+  const contentLength = req.headers.get("content-length");
+  if (contentLength && Number(contentLength) > MAX_FILE_BYTES) {
+    return Response.json({ error: "文件太大" }, { status: 413 });
+  }
+
   let ab: ArrayBuffer;
   try {
     ab = await req.arrayBuffer();
@@ -29,9 +34,9 @@ export async function POST(req: Request): Promise<Response> {
   try {
     const docStr = await unpackDoc(new Uint8Array(ab), secret);
     return Response.json({ docStr });
-  } catch (e) {
+  } catch {
     return Response.json(
-      { error: e instanceof Error ? e.message : "解包失败" },
+      { error: "剧情文件解包失败" },
       { status: 400 },
     );
   }

app/page.tsx

@@ -1406,13 +1406,15 @@ export default function HomePage() {
       setStoryImportError("这个剧情文件是空的。");
       return;
     }
-    if (file.size > 12_000_000) {
+    const isJson = file.name.toLowerCase().endsWith(".json") || file.type === "application/json";
+    const maxImportBytes = isJson ? 12_000_000 : 13_000_000;
+    if (file.size > maxImportBytes) {
       setStoryImportError("剧情文件太大，无法载入。");
       return;
     }
     try {
       let text: string;
-      if (file.name.toLowerCase().endsWith(".json") || file.type === "application/json") {
+      if (isJson) {
         text = await file.text();
       } else {
         const r = await fetch("/api/story-unpack", {
@@ -1551,8 +1553,6 @@ export default function HomePage() {
                 开始
                 <i className="fa-solid fa-arrow-right text-xs" />
               </button>
-            </div>
-            <div className="mt-4 flex flex-col items-center gap-2">
               <input
                 ref={storyImportRef}
                 type="file"
@@ -1563,17 +1563,19 @@ export default function HomePage() {
               <button
                 type="button"
                 onClick={() => storyImportRef.current?.click()}
-                className="inline-flex items-center gap-2 text-[10px] smallcaps text-clay-500 transition-colors hover:text-ember-500"
+                className="group absolute right-[-2.25rem] bottom-2 md:bottom-3 inline-flex items-center justify-center rounded-sm border border-clay-900/20 px-2 py-2 md:py-2.5 text-clay-400 transition-colors hover:border-ember-500 hover:text-ember-500"
               >
-                <i className="fa-solid fa-file-import text-[10px]" />
-                载 · 入 · 剧 · 情
+                <i className="fa-solid fa-file-import text-sm" />
+                <span className="pointer-events-none absolute -bottom-8 left-1/2 -translate-x-1/2 whitespace-nowrap rounded bg-clay-900 px-2 py-1 font-sans text-[11px] text-cream-50 opacity-0 transition-opacity group-hover:opacity-100">
+                  载入剧情
+                </span>
               </button>
-              {storyImportError && (
-                <p className="max-w-[520px] text-center text-xs leading-relaxed text-ember-500">
-                  {storyImportError}
-                </p>
-              )}
             </div>
+            {storyImportError && (
+              <p className="mt-2 text-right text-xs leading-relaxed text-ember-500">
+                {storyImportError}
+              </p>
+            )}
             {prompt && (
               <p className="mt-2 text-right text-xs text-clay-400">
                 Enter 发送 · Shift+Enter 换行

app/play/page.tsx

@@ -1173,6 +1173,9 @@ function PlayInner() {
           const blobUrl = await getOrCreateBlobUrl(first.scene.imageUrl);
           lastImageOriginalUrlRef.current = first.scene.imageUrl;
 
+          const initialStoryState = first.storyStateAfter ?? imported.storyState;
+          if (!initialStoryState) throw new Error("剧情分享文件缺少初始剧情记忆，无法载入。");
+
           const initial: Session = {
             ...imported,
             history: [
@@ -1182,7 +1185,7 @@ function PlayInner() {
                 exit: undefined,
               },
             ],
-            storyState: first.storyStateAfter ?? imported.storyState,
+            storyState: initialStoryState,
             orientation: sessionOrientation,
           };
           replaySourceRef.current = imported;
@@ -1375,7 +1378,7 @@ function PlayInner() {
         !!byoTtsRef.current,
       );
     }
-  }, [currentScene?.id, currentBeat?.id, session?.id]);
+  }, [currentScene?.id, session?.id]);
 
   // Abort all in-flight speculative prefetches when the page unmounts, so we
   // stop paying for background scene/image generation. Empty deps → fires only
@@ -1635,8 +1638,7 @@ function PlayInner() {
         if (recordedNext && recordedNext !== choice.effect.targetBeatId) {
           detachRecordedReplay();
         }
-      }
-      if (
+      } else if (
         replaySourceRef.current &&
         !isRecordedReplayLockedAt(currentBeatRef.current)
       ) {