feat(frontend): reading stats store (TDD) + session heartbeat hook (B0)

frontend/src/lib/useReadingSession.ts

@@ -0,0 +1,32 @@
+import { useEffect, useRef } from "react";
+import { createLocalStats, type StatsStore } from "./readingStats";
+
+const TICK = 30; // 秒：可见期间心跳粒度
+
+/** Reader 页挂一次：可见期间每 30s 落一笔，隐藏/卸载即 flush。 */
+export function useReadingSession(store: StatsStore = createLocalStats()) {
+  const acc = useRef(0);
+  const last = useRef(0);
+  useEffect(() => {
+    last.current = Date.now(); // 渲染期不取时钟，effect 内初始化起算点
+    const flush = () => {
+      const now = Date.now();
+      if (document.visibilityState === "visible") acc.current += (now - last.current) / 1000;
+      last.current = now;
+      if (acc.current >= 1) {
+        store.record(acc.current);
+        acc.current = 0;
+      }
+    };
+    const t = setInterval(flush, TICK * 1000);
+    const onVis = () => flush();
+    document.addEventListener("visibilitychange", onVis);
+    globalThis.addEventListener?.("pagehide", onVis);
+    return () => {
+      clearInterval(t);
+      document.removeEventListener("visibilitychange", onVis);
+      globalThis.removeEventListener?.("pagehide", onVis);
+      flush();
+    };
+  }, [store]);
+}