fix(gitbook): restore useEffect import and drop the orphaned mounted effect - #3857

The docs site has not been deploying. gitbook-pages fails on master at the prerender step:

ReferenceError: useEffect is not defined
Error occurred prerendering page "/ja/integration/claude-code"
Export encountered an error on /[lang]/[...slug]/page, exiting the build.

Why

#1362 removed the mounted state and the mounted && guard on createPortal. That call was right — modal is only built when open is true, open starts false and can only be flipped by a click, so document.body is never reached during SSR. The guard was redundant.

What it missed is that LanguageSwitcher had two effects. It deleted the state, kept the effect that fed it, and rewrote the import to match:

-import { useState, useEffect } from "react";
+import { useState, useLayoutEffect } from "react";
...
-  const [mounted, setMounted] = useState(false);
-  useEffect(() => {
+  useLayoutEffect(() => {
     setMounted(true);
   }, []);

The second useEffect — the body-scroll lock, thirty lines further down — was left untouched and lost its import.

So there are two undefined references, not one:

Restoring the import alone therefore trades a build failure for a runtime one. The effect has had no state to write since #1362; deleting it finishes that commit rather than reviving something nothing reads.

Verification

Real builds, same tree, gitbook/:

no-undef is not enabled by eslint-config-next/core-web-vitals, which is why lint has stayed green on a file that cannot render. Turning it on for just this file:

Nothing else is touched; it is one import and five deleted lines.