fix: enable canvas dev login

nginx.conf

@@ -85,6 +85,15 @@ server {
     proxy_buffering off;
   }
 
+  # Dev/demo auth discovery. The SPA uses this to decide whether to show and
+  # enable the dev-login affordance. Keep this exact route ahead of the SPA
+  # fallback or nginx will return index.html and the client will disable it.
+  location = /internal/dev-login-config {
+    default_type application/json;
+    add_header Cache-Control "no-store" always;
+    return 200 '{"enabled":true,"email":"dev@flow-master.ai"}';
+  }
+
   # SPA fallback to index.html for client-side routing.
   location / {
     try_files $uri $uri/ /index.html;