Serve Access Gate denied HTML as 200 for iframe display.

AppAPI sets frame-ancestors none on 403 proxy responses, which blanked the ExApp iframe; keep API denials as 403.

Co-authored-by: Cursor <cursoragent@cursor.com>

access_gate.go

@@ -177,8 +177,10 @@ func (g AccessGate) normalized() *AccessGate {
 
 func (g *AccessGate) writeDenied(w http.ResponseWriter, r *http.Request) {
 	if acceptsHTML(r.Header.Get("Accept")) {
+		// 200 so AppAPI's proxy CSP keeps frame-ancestors 'self' and the ExApp
+		// iframe can show the message (403 responses get frame-ancestors 'none').
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		w.WriteHeader(http.StatusForbidden)
+		w.WriteHeader(http.StatusOK)
 		_, _ = w.Write(deniedHTML)
 		return
 	}
@@ -216,7 +218,19 @@ var defaultSkipPaths = []string{"/heartbeat", "/enabled", "/init"}
 
 var deniedHTML = []byte(`<!DOCTYPE html>
 <html lang="en">
-<head><meta charset="utf-8"><title>Access denied</title></head>
-<body><h1>Access denied</h1><p>You are not a member of a required group for this app.</p></body>
+<head>
+  <meta charset="utf-8">
+  <title>Access denied</title>
+  <style>
+    :root { font-family: ui-sans-serif, system-ui, sans-serif; color: #1a1a1a; }
+    body { margin: 2rem; max-width: 40rem; }
+    h1 { font-size: 1.4rem; margin-bottom: 0.5rem; }
+    p { color: #444; line-height: 1.5; }
+  </style>
+</head>
+<body>
+  <h1>Access denied</h1>
+  <p>You are not a member of a required group for this app. Ask an administrator to add you to the group if you need access.</p>
+</body>
 </html>
 `)