Add dark mode and theme switching functionality to the application.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 705f2157-ef97-4fbd-89e4-8c7f2ecaea90
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/7ed01c5f-a82d-405a-b728-b2e3d127c60c/69154111-0a6b-4eae-ac42-425c5260b653.jpg

client/src/hooks/use-theme.tsx

@@ -0,0 +1,66 @@
+import { createContext, useContext, useEffect, useState, ReactNode } from 'react';
+
+type Theme = 'dark' | 'light' | 'system';
+
+interface ThemeContextType {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+interface ThemeProviderProps {
+  children: ReactNode;
+  defaultTheme?: Theme;
+  storageKey?: string;
+}
+
+export function ThemeProvider({
+  children,
+  defaultTheme = 'system',
+  storageKey = 'ui-theme',
+  ...props
+}: ThemeProviderProps) {
+  const [theme, setTheme] = useState<Theme>(
+    () => (localStorage.getItem(storageKey) as Theme) || defaultTheme
+  );
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+    root.classList.remove('light', 'dark');
+
+    if (theme === 'system') {
+      const systemTheme = window.matchMedia('(prefers-color-scheme: dark)').matches
+        ? 'dark'
+        : 'light';
+      
+      root.classList.add(systemTheme);
+      return;
+    }
+
+    root.classList.add(theme);
+  }, [theme]);
+
+  const value = {
+    theme,
+    setTheme: (theme: Theme) => {
+      localStorage.setItem(storageKey, theme);
+      setTheme(theme);
+    },
+  };
+
+  return (
+    <ThemeContext.Provider value={value} {...props}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export const useTheme = (): ThemeContextType => {
+  const context = useContext(ThemeContext);
+  
+  if (context === undefined)
+    throw new Error('useTheme must be used within a ThemeProvider');
+  
+  return context;
+};