Set default color scheme to auto in MantineProvider

Added the `defaultColorScheme="auto"` prop to the MantineProvider in App.tsx. This enables automatic color scheme detection, improving the user experience by aligning with the system's settings.

barkmanui/src/App.tsx

@@ -10,7 +10,7 @@ const queryClient = new QueryClient()
 import { MantineProvider } from '@mantine/core';
 
 function App() {
-  return <MantineProvider>{ (
+  return <MantineProvider defaultColorScheme="auto">{ (
       <QueryClientProvider client={queryClient}>
     <>
         <Routes>