feat: add dark/light mode toggle with localStorage persistence

- Enable Tailwind "class" dark mode strategy
- Use CSS custom properties for theme colors (bg, text, border)
- Add ThemeProvider context with toggle and localStorage persistence
- Add Sun/Moon toggle button in the header navigation
- Inline script in index.html prevents FOUC on page load
- All pages (Layout, Login, Register, ProtectedRoute) support both modes
- Default theme follows system preference (prefers-color-scheme)

Closes leeworks-agents/SPARC#33

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/context/ThemeContext.tsx

@@ -0,0 +1,48 @@
+import { createContext, useContext, useEffect, useState } from 'react';
+
+type Theme = 'light' | 'dark';
+
+interface ThemeContextType {
+  theme: Theme;
+  toggleTheme: () => void;
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+function getInitialTheme(): Theme {
+  const stored = localStorage.getItem('theme');
+  if (stored === 'light' || stored === 'dark') return stored;
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+}
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setTheme] = useState<Theme>(getInitialTheme);
+
+  useEffect(() => {
+    const root = document.documentElement;
+    if (theme === 'dark') {
+      root.classList.add('dark');
+    } else {
+      root.classList.remove('dark');
+    }
+    localStorage.setItem('theme', theme);
+  }, [theme]);
+
+  const toggleTheme = () => {
+    setTheme((prev) => (prev === 'dark' ? 'light' : 'dark'));
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (!context) {
+    throw new Error('useTheme must be used within a ThemeProvider');
+  }
+  return context;
+}