Add authentication system with next-auth (CredentialsProvider + JWT)

- Login page with email/password
- Auth middleware (proxy) protecting all routes
- Seed endpoint for admin user creation (admin@optiquestock.com / admin123)
- Session provider wrapping root layout
- User info + logout button in header
- Updated POS sales route to track authenticated user

src/lib/auth.ts

@@ -0,0 +1,58 @@
+import { NextAuthOptions } from 'next-auth'
+import CredentialsProvider from 'next-auth/providers/credentials'
+import bcrypt from 'bcryptjs'
+import { db } from './db'
+
+export const authOptions: NextAuthOptions = {
+  providers: [
+    CredentialsProvider({
+      name: 'credentials',
+      credentials: {
+        email: { label: 'Email', type: 'email' },
+        password: { label: 'Mot de passe', type: 'password' },
+      },
+      async authorize(credentials) {
+        if (!credentials?.email || !credentials?.password) return null
+
+        const employe = await db.employe.findUnique({
+          where: { email: credentials.email },
+        })
+
+        if (!employe || !employe.actif) return null
+
+        const isValid = await bcrypt.compare(credentials.password, employe.motDePasse)
+        if (!isValid) return null
+
+        return {
+          id: employe.id,
+          email: employe.email,
+          name: `${employe.prenom} ${employe.nom}`,
+          role: employe.role,
+        }
+      },
+    }),
+  ],
+  callbacks: {
+    async jwt({ token, user }) {
+      if (user) {
+        token.id = user.id
+        token.role = (user as any).role
+      }
+      return token
+    },
+    async session({ session, token }) {
+      if (session.user) {
+        (session.user as any).id = token.id
+        (session.user as any).role = token.role
+      }
+      return session
+    },
+  },
+  pages: {
+    signIn: '/login',
+  },
+  session: {
+    strategy: 'jwt',
+  },
+  secret: process.env.NEXTAUTH_SECRET,
+}