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.

components.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "new-york",
+  "rsc": false,
+  "tsx": true,
+  "tailwind": {
+    "config": "",
+    "css": "src/styles/main.css",
+    "baseColor": "neutral",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "iconLibrary": "lucide",
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  },
+  "registries": {}
+}