chore: migrate from Tailwind v3 to v4 and upgrade dependencies

Major changes:
- Upgrade Tailwind CSS from v3 to v4 with @tailwindcss/postcss
- Remove deprecated Tailwind v3 configuration and PostCSS plugins
- Upgrade Zod from v3 to v4
- Update React Markdown and other dependencies to latest versions
- Remove @plaiceholder, @headlessui, and other unused dependencies
- Replace SCSS with standard CSS globals
- Add font packages (@fontsource-variable) for better typography
- Consolidate Prettier configuration to .prettierrc
- Remove legacy contact page and custom Payload SCSS
- Add Mantine hooks and Lucide React for improved UI components

postcss.config.mjs

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    "@tailwindcss/postcss": {},
+    ...(process.env.NODE_ENV === "production" ? { cssnano: {} } : {}),
+  },
+};