feat(ui): F0 — Tailwind + design tokens + theme infrastructure

Install Tailwind v3, Radix primitives, cva/tailwind-merge/clsx, lucide-react.
Add HSL design tokens (light + dark) with the Indigo #5b5bf0 accent isolated
to a single --primary token (swappable for a future accent picker), darkMode
'class', anti-flash inline script, useTheme hook, and cn() helper. styles.css
still loads for not-yet-migrated pages. Build green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/main.jsx

@@ -9,6 +9,7 @@ import Unsubscribe from './pages/Unsubscribe.jsx';
 import FolderView from './pages/FolderView.jsx';
 import SearchResults from './pages/SearchResults.jsx';
 import Layout from './components/Layout.jsx';
+import './index.css';
 import './styles.css';
 
 ReactDOM.createRoot(document.getElementById('root')).render(