feat(FRE-596): add PWA setup and responsive design

- Register service worker for offline caching (app shell + API responses)
- Link manifest.json in index.html with updated theme colors
- Update manifest start_url to /app/dashboard for PWA experience
- Add comprehensive team management CSS with responsive breakpoints
- Add alert, loading, and danger button styles
- Mobile-first responsive layout for team list and detail views

src/App.tsx

@@ -4,6 +4,14 @@ import { ClerkProvider } from './lib/auth/clerk-provider';
 import { routes } from './routes';
 import './index.css';
 
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch((err) => {
+      console.warn('Service worker registration failed:', err);
+    });
+  });
+}
+
 render(
   () => (
     <ClerkProvider>