fix tab switching state issues

- Add useFocusEffect to calendar for automatic event reload on tab focus
- Create ChatStore (Zustand) for persistent chat messages across tab switches
- Replace local useState with store in chat screen

apps/client/src/stores/index.ts

@@ -1,2 +1,3 @@
 export { useAuthStore } from "./AuthStore";
+export { useChatStore, type MessageData } from "./ChatStore";
 export { useEventsStore } from "./EventsStore";