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/styles/global.css

@@ -33,6 +33,28 @@ body {
   min-height: 100vh;
 }
 
+.session-gate {
+  min-height: 100vh;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 2rem;
+  background: radial-gradient(ellipse at center, #121a2e 0%, #05070d 70%);
+}
+
+.session-gate-card {
+  width: min(420px, 100%);
+  display: flex;
+  flex-direction: column;
+  gap: 0.75rem;
+  padding: 2rem;
+}
+
+.error-boundary-fallback {
+  padding: 1.5rem;
+  margin: 1rem 0;
+}
+
 a {
   color: var(--accent-blue);
   text-decoration: none;