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/tailwind.config.js

@@ -4,6 +4,7 @@ export default {
     "./index.html",
     "./src/**/*.{js,ts,jsx,tsx}",
   ],
+  darkMode: 'class',
   theme: {
     extend: {
       colors: {
@@ -16,15 +17,15 @@ export default {
         warning: '#f59e0b',
         error: '#ef4444',
         bg: {
-          dark: '#0f172a',
-          card: '#1e293b',
-          'card-hover': '#334155',
+          dark: 'var(--color-bg-dark)',
+          card: 'var(--color-bg-card)',
+          'card-hover': 'var(--color-bg-card-hover)',
         },
         text: {
-          primary: '#f8fafc',
-          secondary: '#94a3b8',
+          primary: 'var(--color-text-primary)',
+          secondary: 'var(--color-text-secondary)',
         },
-        border: '#334155',
+        border: 'var(--color-border)',
       },
     },
   },