feat(frontend): scaffold Tailwind design system + routing + auth callback

- Tailwind v3 + PostCSS + autoprefixer
- BrowserRouter with Layout shell (Header, theme toggle dark/light)
- Pages: HomePage, CallbackPage (SuperOAuth callback handler)
- hooks/useAuth.ts + lib/api.ts (API client base)
- styles/index.css (Tailwind directives)
- Theme persisted in localStorage (od-theme)

frontend/src/main.tsx

@@ -1,5 +1,6 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
+import "./styles/index.css";
 import App from "./App";
 
 createRoot(document.getElementById("root")!).render(