feat: add layout components and pages for NxMesh Admin

- Implemented Header, Sidebar, and Layout components for consistent UI structure.
- Created pages for Agents, Certificates, Configurations, Dashboard, and Settings.
- Integrated React Query for data fetching in Agents, Certificates, Configurations, and Dashboard pages.
- Added Tailwind CSS for styling and responsive design.
- Established a utility function for conditional class names.
- Set up Vite configuration for React project with TypeScript support.
- Included global styles and Tailwind CSS configuration.

frontend/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}