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

package.json

@@ -50,6 +50,7 @@
     "@tanstack/react-query": "^5.82.0",
     "@tanstack/react-table": "^8.21.3",
     "@types/qrcode": "^1.5.6",
+    "bcryptjs": "^3.0.3",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
@@ -84,6 +85,7 @@
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",
+    "@types/bcryptjs": "^3.0.0",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "bun-types": "^1.3.4",