feat(ui): integrate Shadcn UI

This commit integrates the Shadcn UI component library to provide a foundational set of components for the application's
user interface.

Key changes include:
- Added necessary dependencies like `class-variance-authority`, `clsx`, and `tailwind-merge`.
- Initialized Shadcn via its CLI, creating the `components.json` configuration file.
- Configured TypeScript path aliases in `tsconfig.json` and `tsconfig.app.json` to support `@/*` imports.
- Implemented CSS variables for theming, supporting both light and dark modes, in `main.css`.
- Added the standard `cn` utility function for merging Tailwind CSS classes.
- Updated `README.md` with detailed setup instructions for Shadcn UI.

tsconfig.app.json

@@ -5,6 +5,7 @@
   "compilerOptions": {
     "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
 
+    "baseUrl": ".",
     "paths": {
       "@/*": ["./src/*"]
     }