fix(gui): work_plane RGB feed via same-origin proxy

Direct fetch to http://localhost:8877/api/rgb fails from the browser due
to CORS (page is served from :8890). Switch to /api/calibration/rgb.jpg
which presa-mano already proxies same-origin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/work_plane.js

@@ -33,10 +33,8 @@ function updateButtonStates() {
 }
 
 function rgbUrl() {
-  // Nuwa is reachable on host network on port 8877; the GUI is same-origin to
+  // Same-origin RGB proxy exposed by presa-mano (no CORS issues).
-  // presa-mano (8890). Use the upstream host directly.
+  return `/api/calibration/rgb.jpg?t=${Date.now()}`;
-  const host = window.location.hostname;
-  return `http://${host}:8877/api/rgb?t=${Date.now()}`;
 }
 
 function startRgbLoop() {