Add an 18+ entry screen before the app renders

A homescreen-style age gate shown before auth or any route: J621 brand
mark, the explicit 18+ check, an Enter action remembered per browser in
localStorage (j621.age-verified), and a blocked state if the visitor
chooses Leave.

frontend/src/App.tsx

@@ -1,4 +1,4 @@
-import { useEffect, type ReactNode } from "react";
+import { useEffect, useState, type ReactNode } from "react";
 import {
   Navigate,
   Route,
@@ -10,6 +10,7 @@ import {
 import { AppShell } from "@/components/AppShell";
 import { FullPageSpinner } from "@/components/ui";
 import AccountPage from "@/features/account/AccountPage";
+import { AgeGate } from "@/features/age/AgeGate";
 import LoginPage from "@/features/auth/LoginPage";
 import RegisterPage from "@/features/auth/RegisterPage";
 import DetailPage from "@/features/detail/DetailPage";
@@ -20,6 +21,7 @@ import Md5Redirect from "@/features/library/Md5Redirect";
 import OnlinePage from "@/features/online/OnlinePage";
 import UploadPage from "@/features/upload/UploadPage";
 import UsersPage from "@/features/users/UsersPage";
+import { isAgeVerified, markAgeVerified } from "@/lib/age";
 import { useAuth } from "@/store/auth";
 
 function RequireAuth({ children }: { children: ReactNode }) {
@@ -39,11 +41,23 @@ function PostRedirect() {
 export default function App() {
   const ready = useAuth((state) => state.ready);
   const loadUser = useAuth((state) => state.loadUser);
+  const [ageVerified, setAgeVerified] = useState(isAgeVerified);
 
   useEffect(() => {
     void loadUser();
   }, [loadUser]);
 
+  if (!ageVerified) {
+    return (
+      <AgeGate
+        onEnter={() => {
+          markAgeVerified();
+          setAgeVerified(true);
+        }}
+      />
+    );
+  }
+
   if (!ready) {
     return <FullPageSpinner />;
   }