feat(confidence-engine): establish authenticated product boundary

app/layout.jsx

@@ -1,5 +1,6 @@
 import "./globals.css";
 import ThemeToggle from "@/components/theme-toggle";
+import LogoutButton from "@/components/logout-button";
 
 export const metadata = {
   title: "Confidence Engine",
@@ -18,7 +19,10 @@ export default function RootLayout({ children }) {
         <header className="app-chrome border-b border-gray-200/80">
           <div className="mx-auto flex max-w-[1600px] items-center justify-between px-6 py-3">
             <span className="text-sm font-semibold tracking-wide text-teal-700">Confidence Engine</span>
-            <ThemeToggle />
+            <div className="flex items-center gap-2">
+              <ThemeToggle />
+              <LogoutButton />
+            </div>
           </div>
         </header>
         {children}