docs(web-ui/agent): add back-link nav + fix stale doc pointer in screen.html

frontend/packages/agent/src/screen.html

@@ -25,9 +25,8 @@
 <script>
 // Minimal RFB-over-WebSocket renderer. Connects to `screen/ws` on
 // the same host; the harness relays raw RFB bytes to the VNC server
-// running inside the container. See docs/web-ui.md::Per-agent
-// endpoints (GET /screen) for the design rationale + canvas-sizing
-// implementation notes.
+// running inside the container. See docs/web-ui/agent.md (GET /screen)
+// for the design rationale + canvas-sizing implementation notes.
 
 (function () {
   'use strict';