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>

package.json

@@ -38,17 +38,17 @@
     "unist-util-visit": "^5.0.0"
   },
   "devDependencies": {
+    "@tailwindcss/postcss": "^4.1.18",
     "@tailwindcss/typography": "^0.5.19",
     "@types/node": "^24.10.1",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
-    "autoprefixer": "^10.4.22",
     "concurrently": "^9.2.1",
     "eslint": "^9.39.1",
     "eslint-config-next": "^16.0.3",
     "pagefind": "^1.4.0",
     "postcss": "^8.5.6",
-    "tailwindcss": "^3.4.18",
+    "tailwindcss": "^4.1.18",
     "typescript": "^5.9.3"
   }
 }