fix(admin): add basename to BrowserRouter for /admin/ deployment

The router needs basename="/admin" to work correctly when deployed
under the /admin/ subpath.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/admin-client/src/main.tsx

@@ -27,7 +27,7 @@ ReactDOM.createRoot(document.getElementById('root')!).render(
           },
         }}
       >
-        <BrowserRouter>
+        <BrowserRouter basename="/admin">
           <App />
         </BrowserRouter>
       </ConfigProvider>