diff --git a/services/web-ui/public/app.jsx b/services/web-ui/public/app.jsx
index 06e9121..93c7040 100644
--- a/services/web-ui/public/app.jsx
+++ b/services/web-ui/public/app.jsx
@@ -154,4 +154,5 @@ function lighten(hex, amt) {
 }
 
 const root = ReactDOM.createRoot(document.getElementById('root'));
-root.render(<App />);
+const AuthGate = window.AuthGateComponent;
+root.render(<AuthGate><App /></AuthGate>);
diff --git a/services/web-ui/public/auth-gate.jsx b/services/web-ui/public/auth-gate.jsx
new file mode 100644
index 0000000..2e9a0ca
--- /dev/null
+++ b/services/web-ui/public/auth-gate.jsx
@@ -0,0 +1,77 @@
+// auth-gate.jsx — owns the "logged in or not" state.
+//
+// The SPA boots into <AuthGate>, which calls GET /auth/me. On 401 it then
+// calls GET /auth/setup-required and renders <SetupScreen> or <LoginScreen>
+// (defined in screens-auth.jsx, Task 16). On 200 it renders the real <App>.
+//
+// This component is the SINGLE source of truth for the auth check — no other
+// component should redirect to a login page or wipe data on 401. Other code
+// surfaces auth failure by calling window.AuthGate.bounce(), which re-mounts
+// the gate so the next /auth/me request decides what to do.
+
+(function () {
+  const API = '/api/v1';
+  const LAST_PATH_KEY = 'df.auth.last_path';
+
+  async function authFetch(path, opts) {
+    return fetch(API + path, {
+      credentials: 'include',
+      ...opts,
+      headers: {
+        ...(opts && opts.headers),
+        'Content-Type': 'application/json',
+        ...((opts && opts.method && opts.method !== 'GET') ? { 'X-Requested-With': 'dragonflight-ui' } : {}),
+      },
+    });
+  }
+
+  function AuthGate({ children }) {
+    const [state, setState] = React.useState({ kind: 'loading' });
+
+    const check = React.useCallback(async () => {
+      setState({ kind: 'loading' });
+      try {
+        const r = await authFetch('/auth/me');
+        if (r.status === 200) {
+          const me = await r.json();
+          window.ZAMPP_DATA = window.ZAMPP_DATA || {};
+          window.ZAMPP_DATA.ME = me;
+          setState({ kind: 'authed' });
+          return;
+        }
+      } catch (_) { /* fall through to setup/login decision */ }
+      const setup = await authFetch('/auth/setup-required').then(r => r.json()).catch(() => ({ required: false }));
+      setState({ kind: setup.required ? 'setup' : 'login' });
+    }, []);
+
+    React.useEffect(() => { check(); }, [check]);
+
+    // Global hook: anything else (e.g. data.jsx 401 handler) can re-trigger
+    // the gate after auth state changes server-side. Saves current path so
+    // the user is restored to where they were after login.
+    React.useEffect(() => {
+      window.AuthGate = {
+        bounce(reason) {
+          try { sessionStorage.setItem(LAST_PATH_KEY, location.pathname + location.search); } catch {}
+          if (reason) console.warn('[AuthGate] bounce:', reason);
+          check();
+        },
+        signedIn() { check(); },
+      };
+    }, [check]);
+
+    if (state.kind === 'loading') {
+      return (
+        <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'center', height: '100vh', background: 'var(--bg-0)' }}>
+          <div style={{ fontFamily: 'var(--font-mono)', fontSize: 12, color: 'var(--text-3)' }}>Loading…</div>
+        </div>
+      );
+    }
+    if (state.kind === 'setup') return React.createElement(window.SetupScreen, { onDone: () => window.AuthGate.signedIn() });
+    if (state.kind === 'login') return React.createElement(window.LoginScreen, { onDone: () => window.AuthGate.signedIn() });
+    return children;
+  }
+
+  window.AuthGate = window.AuthGate || {};
+  window.AuthGateComponent = AuthGate;
+})();
diff --git a/services/web-ui/public/data.jsx b/services/web-ui/public/data.jsx
index 7842508..af3900c 100644
--- a/services/web-ui/public/data.jsx
+++ b/services/web-ui/public/data.jsx
@@ -62,19 +62,24 @@ window.ZAMPP_DATA = {
 };
 
 async function apiFetch(path, opts = {}) {
+  const method = (opts.method || 'GET').toUpperCase();
+  const headers = {
+    ...(opts.headers || {}),
+    'Content-Type': 'application/json',
+  };
+  if (method !== 'GET' && method !== 'HEAD') headers['X-Requested-With'] = 'dragonflight-ui';
+
   const res = await fetch(API + path, {
     credentials: 'include',
     ...opts,
-    headers: { ...(opts.headers || {}), 'Content-Type': 'application/json' },
+    headers,
   });
-  // 401 from any API call means there's no live session. Bounce to the
-  // login screen instead of leaving the app in a half-loaded state.
-  // While AUTH_ENABLED=false the server returns a synthetic /auth/me with
-  // 200 so this branch never fires; flipping AUTH_ENABLED=true is what
-  // activates the redirect end-to-end.
-  if (res.status === 401 && !location.pathname.endsWith('/login.html')) {
-    location.replace('/login.html');
-    throw new Error('Unauthenticated — redirecting to login');
+  // 401: hand off to AuthGate, which will re-render Login (no full-page reload).
+  if (res.status === 401) {
+    if (window.AuthGate && typeof window.AuthGate.bounce === 'function') {
+      window.AuthGate.bounce('apiFetch saw 401 on ' + path);
+    }
+    throw new Error('Unauthenticated');
   }
   if (!res.ok) throw new Error(res.status + ' ' + res.statusText);
   return res.json();
diff --git a/services/web-ui/public/index.html b/services/web-ui/public/index.html
index 5c8927e..cab074c 100644
--- a/services/web-ui/public/index.html
+++ b/services/web-ui/public/index.html
@@ -25,6 +25,8 @@
 <script src="dist/icons.js"></script>
 <script src="dist/visuals.js"></script>
 <script src="dist/shell.js"></script>
+<script src="dist/auth-gate.js"></script>
+<script src="dist/screens-auth.js"></script>
 <script src="dist/screens-home.js"></script>
 <script src="dist/screens-library.js"></script>
 <script src="dist/screens-asset.js"></script>
diff --git a/services/web-ui/public/shell.jsx b/services/web-ui/public/shell.jsx
index 812cd8e..9ec2591 100644
--- a/services/web-ui/public/shell.jsx
+++ b/services/web-ui/public/shell.jsx
@@ -215,7 +215,7 @@ function Sidebar({ active, onNavigate, me, collapsed, onToggle }) {
           <button className="icon-btn" aria-label="Sign out" data-tip="Sign out" title="Sign out"
             onClick={async () => {
               try { await window.ZAMPP_API.fetch('/auth/logout', { method: 'POST' }); } catch (_) {}
-              window.location.replace('/login.html');
+              window.AuthGate.bounce('user signed out');
             }}>
             <Icon name="power" />
           </button>