fix(membership): allow X-Unibus-* auth headers in CORS preflight

A browser signs every control-plane request with X-Unibus-Pub/Ts/Nonce/Sig
(busauth.signedHeaders). The CORS Allow-Headers only listed Content-Type and
Authorization, so the browser's preflight rejected the real request and the SPA
failed with 'Failed to fetch' on the first authenticated call (listRooms). Add the
four X-Unibus-* headers to Access-Control-Allow-Headers.

This was invisible to the Node smoke (fetch in Node does no CORS preflight); only a
real browser surfaced it. Verified live: enmanuel logs into uniweb against the
cluster and lists rooms. Regression test asserts the header is present.

pkg/membership/server.go

@@ -258,7 +258,10 @@ func (s *Server) applyCORS(w http.ResponseWriter, r *http.Request) (preflight bo
 		// origin. Add (not Set) to preserve any Vary the handler may add later.
 		h.Add("Vary", "Origin")
 		h.Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
-		h.Set("Access-Control-Allow-Headers", "Content-Type, Authorization")
+		// Allow the control-plane request-auth headers a browser client signs every
+		// request with (busauth.signedHeaders), or the browser's CORS preflight blocks
+		// the real request. Content-Type/Authorization stay for JSON bodies.
+		h.Set("Access-Control-Allow-Headers", "Content-Type, Authorization, X-Unibus-Pub, X-Unibus-Ts, X-Unibus-Nonce, X-Unibus-Sig")
 		h.Set("Access-Control-Max-Age", "600")
 	}
 	if r.Method == http.MethodOptions {