fix(frontend): resolve all eslint errors to zero (B1)

frontend/src/readers/PdfReader.tsx

@@ -28,10 +28,17 @@ export default function PdfReader({ book, initialLocator, initialPercent }: Read
   const [err, setErr] = useState("");
   const [nonce, setNonce] = useState(0); // 仅加载重试
   const [redraw, setRedraw] = useState(0); // 仅 resize 重绘(不重新下载文档)
+  // 重新加载(换书/重试)时先清错误:React 官方推荐的「渲染期按下发 key 重置本地 state」,
+  // 替代原先 effect 内的同步 setState(级联渲染);首挂载 key 相等,重置为空操作,行为等价
+  const loadKey = (book.file_url ?? "") + "#" + nonce;
+  const [prevLoadKey, setPrevLoadKey] = useState(loadKey);
+  if (prevLoadKey !== loadKey) {
+    setPrevLoadKey(loadKey);
+    setErr("");
+  }
 
   useEffect(() => {
     let dead = false;
-    setErr("");
     (async () => {
       const buf = await apiRaw(book.file_url ?? "").then((r) => r.arrayBuffer());
       if (dead) return;