agent UI: show 🖥 screen link when gui_enabled

index.html: add hidden #screen-link anchor (/screen, new tab).
app.js: reveal it in refreshState() when api/state returns gui_enabled=true.

gui_enabled is set by the harness when /etc/hyperhive/gui.json exists
(written by the weston VNC service from issue #50). The link opens the
RFB viewer from issue #51 in a new tab.

Closes #52

hive-ag3nt/assets/app.js

@@ -670,6 +670,9 @@
       const s = await resp.json();
       if (!headerSet) { setHeader(s.label, s.dashboard_port); headerSet = true; }
       currentLabel = s.label;
+      // Show the screen link when the weston VNC compositor is enabled.
+      const screenLink = $('screen-link');
+      if (screenLink) screenLink.style.display = s.gui_enabled ? '' : 'none';
       renderTermInput(s.label, s.status === 'online');
       renderInbox(s.inbox || []);
       // Authoritative state comes from the harness via /api/state.