Fix dashboard black screen, add login gate, and expand README.

Pin React Three Fiber to React 18, remove PWA caching, add SessionGate and error boundaries, and document movie fusion, auth, outputs, and troubleshooting.

server/web/src/App.tsx

@@ -1,24 +1,39 @@
+import { lazy, Suspense } from 'react';
 import { Routes, Route, Navigate } from 'react-router-dom';
+import SessionGate from './components/SessionGate';
 import Layout from './components/Layout/Layout';
-import DashboardPage from './pages/DashboardPage';
-import AgentsPage from './pages/AgentsPage';
-import BuilderPage from './pages/BuilderPage';
-import SettingsPage from './pages/SettingsPage';
-import GuidePage from './pages/GuidePage';
+
+const DashboardPage = lazy(() => import('./pages/DashboardPage'));
+const AgentsPage = lazy(() => import('./pages/AgentsPage'));
+const BuilderPage = lazy(() => import('./pages/BuilderPage'));
+const SettingsPage = lazy(() => import('./pages/SettingsPage'));
+const GuidePage = lazy(() => import('./pages/GuidePage'));
+
+function PageFallback() {
+  return (
+    <div className="session-gate" style={{ minHeight: '40vh' }}>
+      <p className="font-tech">Loading…</p>
+    </div>
+  );
+}
 
 function App() {
   return (
+    <SessionGate>
     <Layout>
-      <Routes>
-        <Route path="/" element={<Navigate to="/dashboard" replace />} />
-        <Route path="/dashboard" element={<DashboardPage />} />
-        <Route path="/agents" element={<AgentsPage />} />
-        <Route path="/forge" element={<BuilderPage />} />
-        <Route path="/builder" element={<Navigate to="/forge" replace />} />
-        <Route path="/guide" element={<GuidePage />} />
-        <Route path="/settings" element={<SettingsPage />} />
-      </Routes>
+      <Suspense fallback={<PageFallback />}>
+        <Routes>
+          <Route path="/" element={<Navigate to="/dashboard" replace />} />
+          <Route path="/dashboard" element={<DashboardPage />} />
+          <Route path="/agents" element={<AgentsPage />} />
+          <Route path="/forge" element={<BuilderPage />} />
+          <Route path="/builder" element={<Navigate to="/forge" replace />} />
+          <Route path="/guide" element={<GuidePage />} />
+          <Route path="/settings" element={<SettingsPage />} />
+        </Routes>
+      </Suspense>
     </Layout>
+    </SessionGate>
   );
 }