fix: ssr rendering

src/components/layout/LocaleSwitcher.tsx

@@ -19,6 +19,7 @@ export function LocaleSwitcher() {
 
   return (
     <div className="flex items-center gap-2 w-full py-2 hover:opacity-80">
+      {/* eslint-disable-next-line @next/next/no-img-element */}
       <img
         src={`/flags/${currentLocale}.svg`}
         alt={currentLocale}

src/hooks/useTheme.ts

@@ -1,16 +1,16 @@
 'use client';
 
-import { useEffect, useState } from 'react';
+import { useCallback, useSyncExternalStore } from 'react';
 
 type Theme = 'light' | 'dark';
 
-function getCookie(name: string) {
-  if (typeof document === 'undefined') return null;
+// ── Helpers ──────────────────────────────────────────────────────────────────
 
+function getCookie(name: string): string | null {
+  if (typeof document === 'undefined') return null;
   const value = `; ${document.cookie}`;
   const parts = value.split(`; ${name}=`);
-
-  if (parts.length === 2) return parts.pop()?.split(';').shift();
+  if (parts.length === 2) return parts.pop()?.split(';').shift() ?? null;
   return null;
 }
 
@@ -18,40 +18,58 @@ function setCookie(value: Theme) {
   document.cookie = `theme=${value}; path=/; max-age=31536000`;
 }
 
+function readThemeFromDOM(): Theme {
+  const cookieTheme = getCookie('theme') as Theme | null;
+  if (cookieTheme === 'light' || cookieTheme === 'dark') return cookieTheme;
+  if (document.documentElement.classList.contains('dark')) return 'dark';
+  return 'light';
+}
+
+function applyThemeToDOM(t: Theme) {
+  document.documentElement.classList.toggle('dark', t === 'dark');
+  document.documentElement.classList.toggle('light-mode', t === 'light');
+}
+
+// ── External store ────────────────────────────────────────────────────────────
+// useSyncExternalStore requires a stable subscribe function and a getSnapshot.
+// We use a simple Set of listeners to notify React when the theme changes.
+
+const listeners = new Set<() => void>();
+let clientTheme: Theme | null = null; // lazily initialised on first client read
+
+function getSnapshot(): Theme {
+  if (clientTheme === null) {
+    clientTheme = readThemeFromDOM();
+  }
+  return clientTheme;
+}
+
+// The server always returns the default theme so SSR HTML is deterministic.
+// The client will then reconcile with the real cookie/DOM value via getSnapshot.
+function getServerSnapshot(): Theme {
+  return 'dark';
+}
+
+function subscribe(callback: () => void): () => void {
+  listeners.add(callback);
+  return () => listeners.delete(callback);
+}
+
+function setGlobalTheme(next: Theme) {
+  clientTheme = next;
+  applyThemeToDOM(next);
+  setCookie(next);
+  listeners.forEach((cb) => cb());
+}
+
+// ── Hook ──────────────────────────────────────────────────────────────────────
+
 export function useTheme() {
-  const [theme, setTheme] = useState<Theme>('dark');
+  const theme = useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
 
-  useEffect(() => {
-    const cookieTheme = getCookie('theme') as Theme | null;
-    const rootTheme = document.documentElement.classList.contains('dark')
-      ? 'dark'
-      : 'light';
-
-    const initial = cookieTheme || rootTheme || 'dark';
-
-    setTheme(initial);
-
-    document.documentElement.classList.toggle('dark', initial === 'dark');
-    document.documentElement.classList.toggle(
-      'light-mode',
-      initial === 'light',
-    );
-  }, []);
-
-  function applyTheme(t: Theme) {
-    document.documentElement.classList.toggle('dark', t === 'dark');
-    document.documentElement.classList.toggle('light-mode', t === 'light');
-
-    setCookie(t);
-  }
-
-  function toggleTheme() {
-    setTheme((prev) => {
-      const next = prev === 'dark' ? 'light' : 'dark';
-      applyTheme(next);
-      return next;
-    });
-  }
+  const toggleTheme = useCallback(() => {
+    setGlobalTheme(theme === 'dark' ? 'light' : 'dark');
+  }, [theme]);
 
   return {
     theme,