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/lib/age.ts

@@ -0,0 +1,17 @@
+const AGE_KEY = "j621.age-verified";
+
+export function isAgeVerified(): boolean {
+  try {
+    return window.localStorage.getItem(AGE_KEY) === "1";
+  } catch {
+    return false;
+  }
+}
+
+export function markAgeVerified(): void {
+  try {
+    window.localStorage.setItem(AGE_KEY, "1");
+  } catch {
+    // Private mode with storage disabled: the gate simply shows again.
+  }
+}