feat(phase-18): add in-app notification system (T-128, T-129)

frontend/src/components/Layout.tsx

@@ -2,6 +2,7 @@ import { Outlet } from "react-router-dom";
 import { LogOut } from "lucide-react";
 import { useAuth } from "../context/AuthContext";
 import Sidebar from "./Sidebar";
+import NotificationBell from "./NotificationBell";
 
 export default function Layout() {
   const { user, logout } = useAuth();
@@ -13,6 +14,7 @@ export default function Layout() {
       <div className="flex flex-1 flex-col overflow-hidden">
         {/* Header */}
         <header className="flex h-16 items-center justify-end gap-4 border-b border-gray-800 bg-gray-900 px-6">
+          <NotificationBell />
           <span className="text-sm text-gray-300">{user?.username}</span>
           <button
             onClick={logout}