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/main.tsx

@@ -2,13 +2,28 @@ import React from 'react';
 import ReactDOM from 'react-dom/client';
 import { BrowserRouter } from 'react-router-dom';
 import App from './App';
+import ErrorBoundary from './components/ErrorBoundary';
 import './styles/global.css';
 import './styles/steampunk-theme.css';
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
-    <BrowserRouter>
-      <App />
-    </BrowserRouter>
+    <ErrorBoundary
+      fallback={
+        <div className="session-gate">
+          <div className="session-gate-card card">
+            <h1 className="font-display">AetherForge</h1>
+            <p className="form-hint">The dashboard failed to load. Hard-refresh (Ctrl+Shift+R) or clear site data for this host.</p>
+            <button type="button" className="btn btn-primary" onClick={() => window.location.reload()}>
+              Reload
+            </button>
+          </div>
+        </div>
+      }
+    >
+      <BrowserRouter>
+        <App />
+      </BrowserRouter>
+    </ErrorBoundary>
   </React.StrictMode>
 );