fix(setup): print the hub's public URL, not the host the reader typed

The setup pages build commands that get pasted into a terminal on a *different*
machine, but they built them from location.origin. Open the console by LAN IP or
over a tunnel and the enrolment one-liner told the target machine to curl an
address it may not resolve at all.

/docs and /enroll/:token are now served through a template that stamps in
config.baseUrl — PUBLIC_URL where set, the proxied request host otherwise — with
location.origin left as a fallback for the unsubstituted file. The remaining
127.0.0.1 references are the loopback VNC port on the machine being registered,
which is meant to be literal.

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

public/enroll.html

@@ -119,8 +119,12 @@
 
 <script>
   const token = location.pathname.split('/').filter(Boolean).pop();
-  const origin = location.origin;
-  const link = `${origin}/enroll/${token}`;
+
+  // Substituted by the server with this hub's public address — the commands below
+  // are pasted into a terminal on this machine and have to reach the hub by a name
+  // it can resolve, not by whatever host the reader used to open this page.
+  const stamped = '__HUB__';
+  const origin = stamped.indexOf('__') === 0 ? location.origin : stamped;
 
   // The hub stamps its own address and this token into the install scripts, so
   // each of these really is the whole command — nothing to fill in by hand.