feat(build): integrate Tailwind CSS

This commit integrates Tailwind CSS into the project for utility-first styling.

- Adds `tailwindcss` and `@tailwindcss/vite` as dependencies.
- Configures the Vite build process to use the Tailwind plugin.
- Sets up the main CSS entry point to import Tailwind styles.
- Adds a new route and a `TailwindcssDemo` component to verify the setup.

package.json

@@ -16,7 +16,9 @@
     "format": "prettier --write src/"
   },
   "dependencies": {
+    "@tailwindcss/vite": "^4.1.13",
     "pinia": "^3.0.3",
+    "tailwindcss": "^4.1.13",
     "vue": "^3.5.18",
     "vue-router": "^4.5.1"
   },