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.

src/styles/main.css

@@ -0,0 +1,2 @@
+@import './base.css';
+@import 'tailwindcss';