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/components/Layout.tsx

@@ -1,9 +1,11 @@
 import { Outlet, NavLink, useNavigate } from 'react-router-dom';
 import { useAuth } from '../context/AuthContext';
-import { Search, Layers, BarChart3, Info, Users, LogOut } from 'lucide-react';
+import { useTheme } from '../context/ThemeContext';
+import { Search, Layers, BarChart3, Info, Users, LogOut, Sun, Moon } from 'lucide-react';
 
 export function Layout() {
   const { user, isAdmin, logout } = useAuth();
+  const { theme, toggleTheme } = useTheme();
   const navigate = useNavigate();
 
   const handleLogout = () => {
@@ -23,7 +25,7 @@ export function Layout() {
   }
 
   return (
-    <div className="min-h-screen bg-gradient-to-br from-bg-dark to-indigo-950">
+    <div className="min-h-screen bg-gradient-to-br from-bg-dark to-slate-100 dark:to-indigo-950">
       {/* Header */}
       <header className="bg-bg-card/80 backdrop-blur-lg border-b border-primary/20">
         <div className="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8">
@@ -63,6 +65,13 @@ export function Layout() {
 
             {/* User menu */}
             <div className="flex items-center gap-4">
+              <button
+                onClick={toggleTheme}
+                className="p-2 rounded-lg text-text-secondary hover:text-text-primary hover:bg-bg-card-hover transition-all"
+                aria-label={theme === 'dark' ? 'Switch to light mode' : 'Switch to dark mode'}
+              >
+                {theme === 'dark' ? <Sun size={18} /> : <Moon size={18} />}
+              </button>
               <div className="text-right hidden sm:block">
                 <div className="text-sm font-medium text-text-primary">{user?.email}</div>
                 <div className="text-xs text-text-secondary capitalize">{user?.role}</div>