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/components/auth/SessionProvider.tsx

@@ -0,0 +1,7 @@
+'use client'
+
+import { SessionProvider as NextAuthSessionProvider } from 'next-auth/react'
+
+export default function SessionProvider({ children }: { children: React.ReactNode }) {
+  return <NextAuthSessionProvider>{children}</NextAuthSessionProvider>
+}