refactor(desktop): remove deprecated gatewayStore.ts facade

- Remove gatewayStore.ts (358-line backward-compat facade) that no
  components import from
- All consumers already use domain-specific stores directly
  (connectionStore, agentStore, handStore, etc.)
- Update store/index.ts comment to remove useGatewayStore reference

desktop/src/store/index.ts

@@ -1,8 +1,7 @@
 /**
  * Store Coordinator
  *
- * This module provides a unified interface to all specialized stores,
- * maintaining backward compatibility with components that import useGatewayStore.
+ * This module provides a unified interface to all specialized stores.
  *
  * The coordinator:
  * 1. Injects the shared client into all stores