feat: extract ThemeWatcher component for system theme detection

src/features/provider/ZenProvider.js

@@ -1,10 +1,12 @@
 'use client';
 
 import { ToastProvider, ToastContainer } from '@zen/core/toast';
+import { ThemeWatcher } from '@zen/core/themes';
 
 export function ZenProvider({ children }) {
   return (
     <ToastProvider>
+      <ThemeWatcher />
       {children}
       <ToastContainer />
     </ToastProvider>