agent window: a site's relay stands in for the endpoint it relays

On isokit.buildfor.life, entering https://inference.p0.contact/v1 in the
agent window's settings made the page call the inference server directly,
which browsers refuse (its nginx answers the CORS preflight with 401):
"The browser could not reach ... (Failed to fetch)".

- the relay meta names its upstream (data-upstream; build_web.py
  --agent-upstream, the deploy passes the inference server): that URL now
  goes through the relay and is saved as "follow the site's default"
- an unreachable endpoint says what to do: the relay's URL on a site with
  one, else isokit studio or isokit.buildfor.life
- check_web.py: relayed upstream and the unreachable message

tools/build_web.py

@@ -113,7 +113,9 @@ def main():
     ap = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
     ap.add_argument("--out", default=str(ROOT / "dist" / "web"))
     ap.add_argument("--skip-cargo", action="store_true", help="reuse the last WASM build (front-end changes only)")
-    ap.add_argument("--agent-url", help="default endpoint of the agent window, e.g. a relay on the same site (/llm/v1)")
+    ap.add_argument("--agent-url", help="default endpoint of the agent window: a relay on the same site (/llm/v1)")
+    ap.add_argument("--agent-upstream", default="https://inference.p0.contact/v1",
+                    help="the endpoint that --agent-url relays to; entered in the window, it goes through the relay")
     a = ap.parse_args()
     out = Path(a.out).resolve()
     web = out / "assets" / "web"
@@ -157,7 +159,8 @@ def main():
     html = replace_once(html, '<title>isokit Studio</title>',
                         '<title>isokit Studio</title>\n<meta name="description" content="isokit Studio: edit isometric site plans in the browser.">'
                         + (f'\n<meta name="isokit-build" content="{build}">' if build else "")
-                        + (f'\n<meta name="isokit-agent" content="{html_attr(a.agent_url)}">' if a.agent_url else ""))
+                        + (f'\n<meta name="isokit-agent" content="{html_attr(a.agent_url)}" data-upstream="{html_attr(a.agent_upstream)}">'
+                           if a.agent_url else ""))
     html = replace_once(html, 'href="/assets/studio/favicon.svg"', f'href="assets/studio/favicon.svg?{v}"')
     html = replace_once(html, 'href="/assets/viewer/isokit-view.css"', f'href="assets/viewer/isokit-view.css?{v}"')
     html = replace_once(html, '<link rel="stylesheet" href="/assets/theme.css">', '<style id="iso-theme"></style>')