fix(tailwind): align PostCSS config with Tailwind v4 docs

Root cause: Vercel wasn't processing Tailwind through @tailwindcss/postcss,
serving raw CSS with zero utility classes (23KB theme-only vs 53KB with
utilities). The source() directive was output literally as @media source().

Fixes per official Tailwind v4 + Next.js setup:
- Switch postcss.config.js (CJS) → postcss.config.mjs (ESM)
- Remove autoprefixer (built into Tailwind v4)
- Use plain @import "tailwindcss" (auto source detection)
- Move tailwindcss + @tailwindcss/postcss to dependencies

next/package.json

@@ -20,6 +20,7 @@
     "node": ">=18.17.0"
   },
   "dependencies": {
+    "@tailwindcss/postcss": "^4.2.1",
     "classnames": "^2.5.1",
     "d3-force": "^3.0.0",
     "d3-zoom": "^3.0.0",
@@ -30,6 +31,7 @@
     "minisearch": "^7.1.2",
     "next": "^15.3.0",
     "p5": "1.11.3",
+    "postcss": "^8.5.8",
     "prismjs": "^1.30.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",
@@ -46,11 +48,11 @@
     "remark-html": "^15.0.0",
     "snowball-stemmers": "^0.6.0",
     "stopword": "^3.1.5",
-    "swiper": "^11.2.6"
+    "swiper": "^11.2.6",
+    "tailwindcss": "^4.2.1"
   },
   "devDependencies": {
     "@playwright/test": "^1.55.0",
-    "@tailwindcss/postcss": "^4.1.17",
     "@testing-library/dom": "^10.4.1",
     "@testing-library/jest-dom": "^6.8.0",
     "@testing-library/react": "^16.3.0",
@@ -58,14 +60,11 @@
     "@types/jest": "^30.0.0",
     "@types/node": "24.4.0",
     "@types/react": "^18.2.61",
-    "autoprefixer": "^10.4.22",
     "jest": "^30.1.3",
     "jest-environment-jsdom": "^30.1.2",
     "jest-pnp-resolver": "^1.2.3",
     "next-router-mock": "^1.0.2",
     "playwright": "^1.58.2",
-    "postcss": "^8.5.6",
-    "tailwindcss": "^4.1.17",
     "typescript": "^5.3.3",
     "vercel": "^39"
   },

next/postcss.config.mjs

+const config = {
+  plugins: {
+    "@tailwindcss/postcss": {},
+  },
+};
+export default config;

next/styles/globals.css

-@import "tailwindcss" source("../");
+@import "tailwindcss";
 
 /* ParVagues palette */
 :root {