Migrate to HeroUI v3 and Tailwind CSS v4

- Upgrade from Tailwind CSS v3 to v4 with CSS-first configuration
- Install HeroUI v3 beta packages (@heroui/react, @heroui/styles)
- Migrate PostCSS config to new @tailwindcss/postcss plugin
- Convert tailwind.config.cjs to CSS @theme directive in globals.css
- Replace @tailwindcss/typography with custom prose styles

Component migrations:
- theme-toggle, back-to-top: HeroUI Button with onPress
- post-card, post-list-item: HeroUI Card compound components
- right-sidebar: HeroUI Card, Avatar, Chip
- search-modal: HeroUI Modal with compound structure
- nav-menu: HeroUI Button for mobile controls
- post-list-with-controls: HeroUI Button for sorting/pagination

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

postcss.config.mjs

@@ -0,0 +1,5 @@
+export default {
+  plugins: {
+    "@tailwindcss/postcss": {},
+  },
+};