feat(agent): ship a Node 12 executable for Windows Server 2008 R2

Machines that cannot install Node 22 had no way to run the agent at all, which
left them stuck on direct mode — and direct mode only works when the hub can
route to the VNC port, which it often cannot.

agent.js now resolves the two Node 22 globals it uses through fallbacks: `ws`
for the control and tunnel sockets, and http/https for the single enrolment
POST. Node 22 loads neither, since `globalThis.WebSocket || require('ws')`
short-circuits. The require and the http/https references are static so the
bundler can follow them.

A new Docker stage bundles that file with a Node 12 runtime — the last line
supporting Windows 7 and Server 2008 R2 — into one self-contained .exe, served
from /download/agent.exe and linked from the enrolment page. The route answers
503 rather than 404 in a dev checkout, where the build has not run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

public/enroll.html

@@ -59,6 +59,22 @@
           </li>
         </ol>
 
+        <div class="notice" style="margin-top:14px">
+          <strong>On Windows 7 or Server 2008 R2?</strong>
+          <div class="faint" style="margin-top:6px">
+            Node.js will not install on those — the last version that supported them is long
+            out of date. Use the standalone build instead: one file, nothing to install.
+          </div>
+          <div class="row wrap" style="margin-top:10px">
+            <a class="small" href="/download/agent.exe" download="rcs-agent.exe">Download rcs-agent.exe</a>
+          </div>
+          <div class="faint" style="margin-top:10px">Then in a Command Prompt, in the folder you saved it to:</div>
+          <pre class="code" id="exe-oneliner"></pre>
+          <div class="row wrap" style="margin-top:10px">
+            <button class="small" id="copy-exe">Copy</button>
+          </div>
+        </div>
+
         <p class="login-foot">
           Link expires <span id="expiry"></span> · once enrolled, this machine appears in the operator console
         </p>
@@ -75,6 +91,7 @@
   const unix = `curl -fsSL ${origin}/download/agent.js -o rcs-agent.js \\\n  && node rcs-agent.js enroll ${link} \\\n  && node rcs-agent.js run`;
   const win = `iwr ${origin}/download/agent.js -OutFile rcs-agent.js; `
     + `node rcs-agent.js enroll ${link}; node rcs-agent.js run`;
+  const exe = `rcs-agent.exe enroll ${link}\r\nrcs-agent.exe run`;
 
   function copy(text, button) {
     navigator.clipboard.writeText(text).then(() => {
@@ -87,6 +104,8 @@
   document.getElementById('oneliner').textContent = unix;
   document.getElementById('copy-unix').onclick = (e) => copy(unix, e.currentTarget);
   document.getElementById('copy-win').onclick = (e) => copy(win, e.currentTarget);
+  document.getElementById('exe-oneliner').textContent = exe;
+  document.getElementById('copy-exe').onclick = (e) => copy(exe, e.currentTarget);
 
   fetch(`/api/public/invite/${encodeURIComponent(token)}`)
     .then((r) => r.json().then((body) => ({ ok: r.ok, body })))