fix: add /platform prefix to signIn/signOut redirect URIs

LoginPage and useAuth used window.location.origin without the /platform
base path, causing redirect_uri mismatch with Logto's registered URIs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/auth/useAuth.ts

@@ -7,7 +7,7 @@ export function useAuth() {
   const { currentTenantId } = useOrgStore();
 
   const logout = useCallback(() => {
-    signOut(window.location.origin + '/login');
+    signOut(window.location.origin + '/platform/login');
   }, [signOut]);
 
   return {