feat: add Tailwind CSS and PostCSS configuration

- Created postcss.config.js to configure PostCSS with Tailwind CSS and Autoprefixer.
- Updated svelte.config.js to enable PostCSS preprocessing.
- Added tailwind.config.js for Tailwind CSS configuration.
- Updated yarn.lock to include new dependencies for Tailwind CSS, PostCSS, and related plugins.

tailwind.config.js

@@ -0,0 +1,8 @@
+/** @type {import('tailwindcss').Config} */
+module.exports = {
+    content: ["./frontend/src/**/*.{html,js,svelte,ts}", "./frontend/spa.html"],
+    theme: {
+        extend: {},
+    },
+    plugins: [],
+}