feat: Initial admin dashboard setup with Next.js

[luzhipeng728]

This commit includes the initial setup for a backend management system using Next.js for both frontend and backend.

Key features implemented:

• Next.js project initialized with TypeScript, ESLint, and Tailwind CSS.
• Basic layout with a sidebar navigation and main content area.
• Placeholder pages for Dashboard, Users, and Settings.
• API routes for CRUD operations on a "products" resource (using an in-memory store).
• Frontend pages to list, create, edit, and delete products.
• Basic Tailwind CSS styling for a clean and user-friendly interface.