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>

app/pages/[slug]/page.tsx

@@ -113,7 +113,7 @@ export default async function StaticPage({ params }: Props) {
             <ScrollReveal>
               <article
                 data-toc-content={slug}
-                className="prose prose-lg prose-slate mx-auto max-w-none dark:prose-dark"
+                className="prose prose-lg prose-slate mx-auto max-w-none dark:prose-invert"
               >
                 {page.feature_image && (
                   <div className="-mx-4 mb-8 transition-all duration-500 sm:-mx-12 lg:-mx-20 group-[.toc-open]:lg:-mx-4">