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/internal/api/router.go

@@ -207,7 +207,8 @@ func NewRouter(database *db.Database, wsHub *WSHub, configHandler *ConfigHandler
 					return
 				}
 
-				// SPA fallback - serve index.html
+				// SPA fallback - serve index.html (never cache: hashed assets change each build)
+				w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate")
 				http.ServeFile(w, r, filepath.Join(webRoot, "index.html"))
 			})
 		} else {