Migrate to Tailwind CSS v4 with CSS-first configuration

- Replace tailwindcss v3 + autoprefixer with tailwindcss v4 + @tailwindcss/postcss
- Migrate tailwind.config.cjs theme to @theme block in globals.css
- Add @custom-variant dark for class-based dark mode (next-themes)
- Load typography plugin via @plugin directive, replace prose-dark with prose-invert
- Convert prose dark mode overrides from JS config to CSS (.dark .prose rules)
- Add @source directive for content submodule detection
- Replace postcss.config.cjs with postcss.config.mjs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

postcss.config.mjs

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