fix: cache hosted pairing request at module level to survive remounts

readHostedPairingRequest() was called inside useRef() which re-initializes
on remount. After the first mount, stripPairingTokenFromUrl() removes the
token from the URL, so any subsequent remount (React Strict Mode, Suspense,
parent key change) would read null from the stripped URL and show a
misleading 'missing host or token' error.

Cache the result in a module-level variable so it persists across remounts.

Author: cursoragent

apps/web/src/components/auth/PairingRouteSurface.tsx

@@ -161,8 +161,16 @@ export function PairingRouteSurface({
   );
 }
 
+let cachedHostedPairingRequest: ReturnType<typeof readHostedPairingRequest> | undefined;
+function readHostedPairingRequestOnce() {
+  if (cachedHostedPairingRequest === undefined) {
+    cachedHostedPairingRequest = readHostedPairingRequest();
+  }
+  return cachedHostedPairingRequest;
+}
+
 export function HostedPairingRouteSurface() {
-  const hostedPairingRequestRef = useRef(readHostedPairingRequest());
+  const hostedPairingRequestRef = useRef(readHostedPairingRequestOnce());
   const [status, setStatus] = useState<"pairing" | "paired" | "error">("pairing");
   const [message, setMessage] = useState("Connecting to this backend.");
   const submitAttemptedRef = useRef(false);