fix(css): downgrade Tailwind CSS from v4 to v3.4 for better browser compatibility

Tailwind CSS v4 requires modern CSS features (@layer, @property, color-mix)
that are not supported in older WebView2 versions, causing styles to fail
on some Windows 11 systems.

Changes:
- Replace @tailwindcss/vite with postcss + autoprefixer
- Downgrade tailwindcss from 4.1.13 to 3.4.17
- Convert CSS imports from v4 syntax to v3 @tailwind directives
- Add darkMode: "selector" to tailwind.config.js
- Create postcss.config.js for PostCSS pipeline

This improves compatibility with older browsers and WebView2 runtimes
while maintaining the same visual appearance.

postcss.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};