feat: refactor theme management and remove next-themes dependency

- Replaced the next-themes package with a custom ThemeProvider component for enhanced theme management.
- Implemented local storage handling for theme persistence and system theme detection.
- Updated the Toaster component to utilize the new theme context.
- Removed unused next-themes references from package.json and package-lock.json.

package.json

@@ -20,7 +20,6 @@
     "clsx": "^2.1.1",
     "lucide-react": "^1.11.0",
     "next": "16.2.4",
-    "next-themes": "^0.4.6",
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "shadcn": "^4.5.0",