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

CLAUDE.md

@@ -90,6 +90,7 @@ src/
 └── stores/                     # Zustand state management
     ├── index.ts                # Re-exports all stores
     ├── AuthStore.ts            # user, token, isAuthenticated, login(), logout(), setToken()
+    ├── ChatStore.ts            # messages[], addMessage(), updateMessage(), clearMessages()
     └── EventsStore.ts          # events[], setEvents(), addEvent(), updateEvent(), deleteEvent()
 ```
 
@@ -298,7 +299,9 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
   - Orange dot indicator for days with events
   - Tap-to-open modal overlay showing EventCards for selected day
   - Supports events from adjacent months visible in grid
+  - Uses `useFocusEffect` for automatic reload on tab focus
 - Chat screen functional with FlashList, message sending, and event confirm/reject
+  - Messages persisted via ChatStore (survives tab switches)
 - `ApiClient`: get(), post(), put(), delete() implemented
 - `EventService`: getAll(), getById(), getByDateRange(), create(), update(), delete() - fully implemented
 - `ChatService`: sendMessage(), confirmEvent(convId, msgId, action, event?, eventId?, updates?), rejectEvent() - supports create/update/delete actions
@@ -306,6 +309,7 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
 - `ProposedEventCard`: Displays proposed events (title, date, description, recurring indicator) with confirm/reject buttons
 - `Themes.tsx`: Centralized color definitions including textPrimary, borderPrimary, eventIndicator
 - `EventsStore`: Zustand store with setEvents(), addEvent(), updateEvent(), deleteEvent() - stores ExpandedEvent[]
+- `ChatStore`: Zustand store with addMessage(), updateMessage(), clearMessages() - persists messages across tab switches
 - Auth screens (Login, Register), Event Detail, and Note screens exist as skeletons
 - AuthStore defined with `throw new Error('Not implemented')`