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/layout.tsx

@@ -107,10 +107,10 @@ export default function RootLayout() {
   // Show loading state while checking auth
   if (isChecking) {
     return (
-      <div className="flex justify-center items-center min-h-screen bg-gray-50">
+      <div className="flex justify-center items-center min-h-screen bg-gray-50 dark:bg-neutral-950">
         <div className="text-center">
           <div className="inline-block animate-spin rounded-full h-12 w-12 border-b-2 border-primary-600 mb-4"></div>
-          <p className="text-gray-600">Loading...</p>
+          <p className="text-gray-600 dark:text-neutral-400">Loading...</p>
         </div>
       </div>
     );