's'

app/context/ThemeContext.tsx

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+"use client";
+
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useState,
+  useCallback,
+  type ReactNode,
+} from "react";
+
+const STORAGE_KEY = "theme";
+
+export type Theme = "light" | "dark";
+
+type ThemeContextValue = {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  toggleTheme: () => void;
+};
+
+const ThemeContext = createContext<ThemeContextValue | null>(null);
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>("light");
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setMounted(true);
+    const hasDark = document.documentElement.classList.contains("dark");
+    setThemeState(hasDark ? "dark" : "light");
+  }, []);
+
+  useEffect(() => {
+    if (!mounted) return;
+    const root = document.documentElement;
+    if (theme === "dark") {
+      root.classList.add("dark");
+    } else {
+      root.classList.remove("dark");
+    }
+    localStorage.setItem(STORAGE_KEY, theme);
+  }, [theme, mounted]);
+
+  const setTheme = useCallback((newTheme: Theme) => {
+    setThemeState(newTheme);
+  }, []);
+
+  const toggleTheme = useCallback(() => {
+    setThemeState((prev) => (prev === "light" ? "dark" : "light"));
+  }, []);
+
+  const value: ThemeContextValue = {
+    theme,
+    setTheme,
+    toggleTheme,
+  };
+
+  return (
+    <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const ctx = useContext(ThemeContext);
+  if (!ctx) throw new Error("useTheme must be used within ThemeProvider");
+  return ctx;
+}