fix: downgrade Next.js 16 to 15 for Vercel deployment compatibility

Next.js 16.1.6 is not yet fully supported by Vercel's build system,
causing 404 on all routes. Downgraded to Next.js 15.5.12 + React 18,
fixed ESLint config, TypeScript config, and lint errors.

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

lib/utils.ts

@@ -1,8 +1,8 @@
-import { type ClassValue, clsx } from "clsx";
+import { clsx, type ClassValue } from "clsx";
 
 // Utilitaire pour combiner les classes CSS (compatible Tailwind)
 export function cn(...inputs: ClassValue[]) {
-  return inputs.filter(Boolean).join(" ");
+  return clsx(inputs);
 }
 
 // Formater un prix en euros