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

app/(protected)/formations/[moduleId]/page.tsx

@@ -1,3 +1,4 @@
+import Link from "next/link";
 import { redirect } from "next/navigation";
 import { createClient } from "@/lib/supabase/server";
 import Card from "@/components/ui/Card";
@@ -40,12 +41,12 @@ export default async function ModulePage({ params }: ModulePageProps) {
     <div className="max-w-4xl">
       {/* Breadcrumb */}
       <nav className="flex items-center gap-2 text-sm mb-8">
-        <a
+        <Link
           href="/formations"
           className="text-white/40 hover:text-white transition-colors"
         >
           Formations
-        </a>
+        </Link>
         <span className="text-white/20">/</span>
         <span className="text-white/40">Semaine {module.week_number}</span>
         <span className="text-white/20">/</span>
@@ -150,7 +151,7 @@ export default async function ModulePage({ params }: ModulePageProps) {
 
       {/* Actions */}
       <div className="flex items-center justify-between">
-        <a
+        <Link
           href="/formations"
           className="text-white/40 hover:text-white text-sm transition-colors flex items-center gap-1"
         >
@@ -168,7 +169,7 @@ export default async function ModulePage({ params }: ModulePageProps) {
             />
           </svg>
           Retour aux formations
-        </a>
+        </Link>
 
         <MarkCompleteButton
           moduleId={moduleId}