feat(adminsite): test harness, typed client and the not-connected state

The repo's first frontend test setup: Vitest, React Testing Library, jsdom.
Scoped to the flows that lose money or leak data when broken, per spec 4.

lib/api.ts collapses every failure into three the UI can act on:
NotConnected (unreachable, or no URL baked in), ApiError 401 (redirect), and
ApiError with the backend's own message, which is customer-facing and shown
verbatim rather than replaced with something vaguer.

The not-connected panel names the variable, the value baked in, and both
reasons it fails -- unreachable from the browser, or missing from admin's
ADMIN_ORIGIN. Proven by test before it existed.

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

adminsite/components/NotConnected.tsx

@@ -0,0 +1,28 @@
+/*
+ * The deployment failure this repo makes most often, made legible. It names the
+ * variable, the value baked in, and both reasons it fails — unreachable from
+ * the browser, or missing from admin's ADMIN_ORIGIN.
+ */
+export function NotConnectedPanel({ url }: { url: string }) {
+    return (
+        <div className="grid max-w-2xl gap-3 rounded border border-expired bg-panel p-5">
+            <h2 className="text-xl text-expired">Not connected to the licensing service</h2>
+            {url ? (
+                <p className="text-ink-2">
+                    This build points at <code className="text-ink">ADMIN_API_URL</code> ={" "}
+                    <code className="text-ink">{url}</code>, which did not respond.
+                </p>
+            ) : (
+                <p className="text-ink-2">
+                    <code className="text-ink">ADMIN_API_URL</code> was not set when this app was
+                    built, so there is nowhere to send requests.
+                </p>
+            )}
+            <p className="text-[0.82rem] text-ink-3">
+                The value is baked in when the image is built and has to be reachable from your
+                browser, not just from the server. It also has to appear in the licensing
+                service&rsquo;s <code>ADMIN_ORIGIN</code>, or the browser blocks every request.
+            </p>
+        </div>
+    );
+}