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/package.json

@@ -18,6 +18,9 @@
     "@types/react": "^18.3.1",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.2.1",
+    "autoprefixer": "^10.4.27",
+    "postcss": "^8.5.8",
+    "tailwindcss": "^3.4.19",
     "typescript": "^5.4.3",
     "vite": "^5.2.8"
   }