fix: Sign out button in settings dropdown not working

The header dropdown 'Sign out' menu item used onclick='handleLogout()'
but handleLogout was never defined as a global function.

Added window.handleLogout in auth.js that delegates to
AuthManager.logout(), same as the existing logout-btn handler.

frontend/js/auth.js

@@ -201,6 +201,9 @@ const AuthManager = {
     const logoutBtn = document.getElementById("logout-btn");
     if (logoutBtn) logoutBtn.addEventListener("click", () => AuthManager.logout());
 
+    // Also wire up the header dropdown "Sign out" (onclick="handleLogout()")
+    window.handleLogout = () => AuthManager.logout();
+
     const adminRow = document.getElementById("admin-menu-row");
     if (adminRow) {
       if (user.role === "admin") {