fix(eslint): resolve setState-in-effect errors in dashboard components (#1362)

- LanguageSwitcher: remove mounted state + useLayoutEffect pattern
  Portal renders directly based on open state (SSR-safe without client check)
- UsageStats: replace stats-null check with isInitialLoad ref to avoid
  setState in effect body (cascading render issue)

gitbook/components/LanguageSwitcher.js

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useEffect } from "react";
+import { useState, useLayoutEffect } from "react";
 import { createPortal } from "react-dom";
 import { useRouter, usePathname } from "next/navigation";
 import { Globe, X } from "lucide-react";
@@ -15,12 +15,11 @@ function extractLangFromPath(pathname) {
 
 export default function LanguageSwitcher({ currentLang }) {
   const [open, setOpen] = useState(false);
-  const [mounted, setMounted] = useState(false);
   const router = useRouter();
   const pathname = usePathname();
   const current = getLanguage(currentLang);
 
-  useEffect(() => {
+  useLayoutEffect(() => {
     setMounted(true);
   }, []);
 
@@ -91,7 +90,7 @@ export default function LanguageSwitcher({ currentLang }) {
         <span className="sm:hidden">{current.flag}</span>
       </button>
 
-      {mounted && open && createPortal(modal, document.body)}
+      {open && createPortal(modal, document.body)}
     </>
   );
 }

src/shared/components/UsageStats.js

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState, useEffect, useMemo, useCallback } from "react";
+import { useState, useEffect, useMemo, useCallback, useRef } from "react";
 import { useSearchParams, useRouter } from "next/navigation";
 import { FREE_PROVIDERS, AI_PROVIDERS } from "@/shared/constants/providers";
 
@@ -203,6 +203,7 @@ export default function UsageStats({ period: periodProp, setPeriod: setPeriodPro
   const [viewMode, setViewMode] = useState("costs");
   const [providers, setProviders] = useState([]);
   const [periodLocal, setPeriodLocal] = useState("today");
+  const isInitialLoad = useRef(true);
   const period = periodProp ?? periodLocal;
   const setPeriod = setPeriodProp ?? setPeriodLocal;
 
@@ -231,8 +232,12 @@ export default function UsageStats({ period: periodProp, setPeriod: setPeriodPro
   // Fetch filtered stats via REST when period changes
   useEffect(() => {
     // First load: show full spinner; subsequent: show subtle fetching indicator
-    if (!stats) setLoading(true);
-    else setFetching(true);
+    if (isInitialLoad.current) {
+      isInitialLoad.current = false;
+      setLoading(true);
+    } else {
+      setFetching(true);
+    }
 
     fetch(`/api/usage/stats?period=${period}`)
       .then((r) => r.ok ? r.json() : null)