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/.env.example

@@ -0,0 +1,3 @@
+# URL complète d'autorisation OAuth — SuperOAuth
+# Format : https://superoauth.tetardtek.com/oauth/authorize?client_id=XXX&redirect_uri=http://localhost:5173/callback&response_type=token
+VITE_SUPEROAUTH_AUTHORIZE_URL=