feat: add dark/light theme switching with ThemeContext and localStorage persistence

frontend/src/ThemeContext.tsx

@@ -0,0 +1,46 @@
+import React, { createContext, useContext, useEffect, useState } from 'react';
+
+type Theme = 'light' | 'dark';
+
+interface ThemeContextType {
+  theme: Theme;
+  toggleTheme: () => void;
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+export const ThemeProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
+  const [theme, setTheme] = useState<Theme>(() => {
+    const saved = localStorage.getItem('theme');
+    if (saved === 'light' || saved === 'dark') return saved;
+    if (window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches) {
+      return 'light';
+    }
+    return 'dark';
+  });
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+    root.classList.remove('light', 'dark');
+    root.classList.add(theme);
+    localStorage.setItem('theme', theme);
+  }, [theme]);
+
+  const toggleTheme = () => {
+    setTheme(prev => (prev === 'dark' ? 'light' : 'dark'));
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+};
+
+export const useTheme = () => {
+  const context = useContext(ThemeContext);
+  if (context === undefined) {
+    throw new Error('useTheme must be used within a ThemeProvider');
+  }
+  return context;
+};