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/index.css

@@ -2,6 +2,26 @@
 @tailwind components;
 @tailwind utilities;
 
+/* Light mode (default) */
+:root {
+  --color-bg-dark: #f1f5f9;
+  --color-bg-card: #ffffff;
+  --color-bg-card-hover: #e2e8f0;
+  --color-text-primary: #0f172a;
+  --color-text-secondary: #475569;
+  --color-border: #cbd5e1;
+}
+
+/* Dark mode */
+.dark {
+  --color-bg-dark: #0f172a;
+  --color-bg-card: #1e293b;
+  --color-bg-card-hover: #334155;
+  --color-text-primary: #f8fafc;
+  --color-text-secondary: #94a3b8;
+  --color-border: #334155;
+}
+
 body {
   font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
   -webkit-font-smoothing: antialiased;
@@ -15,7 +35,7 @@ body {
 }
 
 ::-webkit-scrollbar-track {
-  background: #1e293b;
+  background: var(--color-bg-card);
 }
 
 ::-webkit-scrollbar-thumb {
@@ -30,5 +50,5 @@ body {
 /* Selection */
 ::selection {
   background: rgba(99, 102, 241, 0.3);
-  color: #f8fafc;
+  color: var(--color-text-primary);
 }