fix(#1988): send agent username not root for vnc apple-dh auth

frontend/packages/agent/src/screen.html

@@ -128,6 +128,21 @@
     setTimeout(() => { msg.style.opacity = '0'; }, 2500);
   }
 
+  // --- Agent username (for Apple-DH auth) ---
+  // weston runs the VNC backend as the agent's own non-root unix user
+  // (weston-vnc.nix: `User = userName`), and neatvnc's Apple-DH path
+  // does getpwnam(username) + requires pw_uid == weston's uid before PAM
+  // is ever consulted — so the auth username MUST be that agent's name,
+  // not a hardcoded "root". `/api/state` exposes it as `label` (same
+  // path-relative `document.baseURI` trick as the WS below, so it works
+  // under the nginx /agent/<name>/ prefix too). Kicked off here so it
+  // resolves concurrently with the RFB handshake; the Apple-DH step
+  // awaits it.
+  const agentLabelPromise = fetch(new URL('api/state', document.baseURI))
+    .then((r) => (r.ok ? r.json() : null))
+    .then((s) => (s && s.label) || null)
+    .catch(() => null);
+
   // --- WebSocket connection ---
   // Path-relative URL — `document.baseURI` resolves against the
   // page's URL so the agent page mounted under a nginx prefix
@@ -436,11 +451,16 @@
             // weston's vnc_handle_auth (libweston/backend-vnc/vnc.c) does
             // getpwnam(username) and requires pw_uid == weston's own uid
             // BEFORE PAM is ever consulted — an empty/garbage username is
-            // rejected outright. weston runs as root, so the username must
+            // rejected outright. Our weston runs as the agent's own
-            // be "root". The password stays empty; pam_permit.so on the
+            // non-root user (weston-vnc.nix forbids root), so the username
+            // must be that agent's name — resolved from /api/dashboard-state
+            // above. The password stays empty; pam_permit.so on the
             // weston-remote-access PAM service accepts it.
+            const username = (await agentLabelPromise) || 'root';
+            dbg('Apple-DH: auth username=' + username +
+                (username === 'root' ? ' (label unresolved — falling back)' : ''));
             const creds = new Uint8Array(128);
-            creds.set(new TextEncoder().encode('root'), 0);
+            creds.set(new TextEncoder().encode(username), 0);
             const encCreds = await aes128ecb(aesKey, creds);
 
             // Send: encrypted_creds + client_pub