feat(hackathon): add dark mode support with theme toggle

- Add ThemeProvider with system/light/dark mode support and localStorage persistence
- Add ThemeToggle component for pages without sidebar (landing, login, signup)
- Enable class-based dark mode in Tailwind CSS v4 via @custom-variant
- Add dark mode classes to landing page, login, signup, and all dashboard pages
- Keep IMPHNEN logo blue in dark mode, invert Kolosal logo

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/hackathon/src/app/users/[userId]/layout.tsx

@@ -6,21 +6,21 @@ const UserDetailLayout: FC = (): ReactElement => {
   const [sidebarOpen, setSidebarOpen] = useState(false);
 
   return (
-    <div className="flex min-h-screen bg-gray-50">
+    <div className="flex min-h-screen bg-gray-50 dark:bg-neutral-950">
       <Sidebar isOpen={sidebarOpen} onClose={() => setSidebarOpen(false)} />
 
       <div className="flex-1 flex flex-col overflow-auto">
         {/* Mobile Header with Hamburger */}
-        <div className="lg:hidden bg-white border-b px-4 py-3 flex items-center">
+        <div className="lg:hidden bg-white dark:bg-neutral-900 border-b dark:border-neutral-700 px-4 py-3 flex items-center">
           <button
             onClick={() => setSidebarOpen(true)}
-            className="p-2 rounded-lg hover:bg-gray-100 transition-colors"
+            className="p-2 rounded-lg hover:bg-gray-100 dark:hover:bg-neutral-800 transition-colors"
           >
-            <svg className="w-6 h-6 text-gray-600" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <svg className="w-6 h-6 text-gray-600 dark:text-neutral-400" fill="none" stroke="currentColor" viewBox="0 0 24 24">
               <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 6h16M4 12h16M4 18h16" />
             </svg>
           </button>
-          <h1 className="ml-3 text-lg font-bold text-gray-900">🏆 Hackathon</h1>
+          <h1 className="ml-3 text-lg font-bold text-gray-900 dark:text-white">Hackathon</h1>
         </div>
 
         <main className="flex-1 overflow-auto">