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>

next-env.d.ts

@@ -1,6 +1,6 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-import "./.next/types/routes.d.ts";
+import "./.next/dev/types/routes.d.ts";
 
 // NOTE: This file should not be edited
 // see https://nextjs.org/docs/app/api-reference/config/typescript for more information.