feat(frontend): shelf reading stats card + session tracking (B4)

frontend/src/pages/Shelf.tsx

@@ -17,6 +17,7 @@ import { useAuth } from "@/auth/AuthContext";
 import { Cover } from "@/components/Cover";
 import { toast } from "@/components/Toaster";
 import { UploadButton } from "@/components/upload-button";
+import { ReadingStatsCard } from "@/components/reading-stats-card";
 import { formatBadge } from "@/components/ui";
 import {
   AlertDialog,
@@ -280,6 +281,8 @@ export default function Shelf() {
 
       {lib === LIB0 && <p className="sr-only" aria-live="polite">{`当前筛选:${libName}`}</p>}
 
+      <ReadingStatsCard />
+
       {(libsQ.data ?? []).length === 0 && !libsQ.isPending && !searching && (
         <p className="mb-6 flex items-center gap-2 rounded-md border border-accent/30 bg-accent/10 px-3 py-2 text-sm text-accent-strong">
           <LibraryBig className="size-4 shrink-0" />