Add theme toggle with light, dark, and system support

- **Header:** Integrate `ThemeToggle` component into the user menu area.
- **Theme Provider:** Introduce `ThemeProvider` context for managing and persisting theme preferences.
- **New Components:** Add `ThemeToggle` for switching themes and `ThemeProvider` to handle state and system preferences.
- Ensure responsive updates and localStorage persistence for user-selected themes.

frontend/src/components/layout/Header.tsx

@@ -22,6 +22,7 @@ import { Button } from '@/components/ui/button';
 import { Avatar, AvatarFallback } from '@/components/ui/avatar';
 import { Settings, LogOut, User, Shield } from 'lucide-react';
 import { cn } from '@/lib/utils';
+import { ThemeToggle } from '@/components/theme';
 
 /**
  * Get user initials for avatar
@@ -97,8 +98,9 @@ export function Header() {
           </nav>
         </div>
 
-        {/* Right side - User menu */}
-        <div className="ml-auto flex items-center space-x-4">
+        {/* Right side - Theme toggle and user menu */}
+        <div className="ml-auto flex items-center space-x-2">
+          <ThemeToggle />
           <DropdownMenu>
             <DropdownMenuTrigger asChild>
               <Button variant="ghost" className="relative h-10 w-10 rounded-full">

frontend/src/components/theme/ThemeProvider.tsx

@@ -0,0 +1,83 @@
+/**
+ * Theme Provider
+ * Manages light/dark mode with persistence
+ */
+
+'use client';
+
+import { createContext, useContext, useEffect, useState } from 'react';
+
+type Theme = 'light' | 'dark' | 'system';
+
+interface ThemeContextValue {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  resolvedTheme: 'light' | 'dark';
+}
+
+const ThemeContext = createContext<ThemeContextValue | undefined>(undefined);
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>('system');
+  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');
+
+  // Initialize theme from localStorage on mount
+  useEffect(() => {
+    const stored = localStorage.getItem('theme') as Theme | null;
+    if (stored && ['light', 'dark', 'system'].includes(stored)) {
+      setThemeState(stored);
+    }
+  }, []);
+
+  // Apply theme to document and resolve system preference
+  useEffect(() => {
+    const root = window.document.documentElement;
+    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+
+    const applyTheme = () => {
+      let effectiveTheme: 'light' | 'dark';
+
+      if (theme === 'system') {
+        effectiveTheme = mediaQuery.matches ? 'dark' : 'light';
+      } else {
+        effectiveTheme = theme;
+      }
+
+      setResolvedTheme(effectiveTheme);
+
+      root.classList.remove('light', 'dark');
+      root.classList.add(effectiveTheme);
+    };
+
+    applyTheme();
+
+    // Listen for system theme changes
+    const handleChange = () => {
+      if (theme === 'system') {
+        applyTheme();
+      }
+    };
+
+    mediaQuery.addEventListener('change', handleChange);
+    return () => mediaQuery.removeEventListener('change', handleChange);
+  }, [theme]);
+
+  const setTheme = (newTheme: Theme) => {
+    setThemeState(newTheme);
+    localStorage.setItem('theme', newTheme);
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, setTheme, resolvedTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (!context) {
+    throw new Error('useTheme must be used within ThemeProvider');
+  }
+  return context;
+}

frontend/src/components/theme/ThemeToggle.tsx

@@ -0,0 +1,51 @@
+/**
+ * Theme Toggle Button
+ * Switches between light, dark, and system themes
+ */
+
+'use client';
+
+import { Moon, Sun, Monitor } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from '@/components/ui/dropdown-menu';
+import { useTheme } from './ThemeProvider';
+
+export function ThemeToggle() {
+  const { theme, setTheme, resolvedTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="ghost" size="icon" aria-label="Toggle theme">
+          {resolvedTheme === 'dark' ? (
+            <Moon className="h-5 w-5" />
+          ) : (
+            <Sun className="h-5 w-5" />
+          )}
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme('light')}>
+          <Sun className="mr-2 h-4 w-4" />
+          Light
+          {theme === 'light' && <span className="ml-auto">✓</span>}
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme('dark')}>
+          <Moon className="mr-2 h-4 w-4" />
+          Dark
+          {theme === 'dark' && <span className="ml-auto">✓</span>}
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme('system')}>
+          <Monitor className="mr-2 h-4 w-4" />
+          System
+          {theme === 'system' && <span className="ml-auto">✓</span>}
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}

frontend/src/components/theme/index.ts

@@ -0,0 +1,6 @@
+/**
+ * Theme components
+ */
+
+export { ThemeProvider, useTheme } from './ThemeProvider';
+export { ThemeToggle } from './ThemeToggle';