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/app/(tabs)/chat.tsx

@@ -5,6 +5,7 @@ import Header from "../../components/Header";
 import BaseBackground from "../../components/BaseBackground";
 import { FlashList } from "@shopify/flash-list";
 import { ChatService } from "../../services";
+import { useChatStore, MessageData } from "../../stores";
 import { ProposedEventChange } from "@caldav/shared";
 import { ProposedEventCard } from "../../components/ProposedEventCard";
 
@@ -24,17 +25,12 @@ type ChatMessageProps = {
   onReject?: () => void;
 };
 
-type MessageData = ChatMessageProps & {
-  id: string;
-  conversationId?: string;
-};
-
 type ChatInputProps = {
   onSend: (text: string) => void;
 };
 
 const Chat = () => {
-  const [messages, setMessages] = useState<MessageData[]>([]);
+  const { messages, addMessage, updateMessage } = useChatStore();
 
   const handleEventResponse = async (
     action: "confirm" | "reject",
@@ -43,11 +39,7 @@ const Chat = () => {
     proposedChange?: ProposedEventChange,
   ) => {
     // Mark message as responded (optimistic update)
-    setMessages((prev) =>
-      prev.map((msg) =>
-        msg.id === messageId ? { ...msg, respondedAction: action } : msg,
-      ),
-    );
+    updateMessage(messageId, { respondedAction: action });
 
     try {
       const response =
@@ -68,15 +60,11 @@ const Chat = () => {
         content: response.message.content,
         conversationId: response.conversationId,
       };
-      setMessages((prev) => [...prev, botMessage]);
+      addMessage(botMessage);
     } catch (error) {
       console.error(`Failed to ${action} event:`, error);
       // Revert on error
-      setMessages((prev) =>
-        prev.map((msg) =>
-          msg.id === messageId ? { ...msg, respondedAction: undefined } : msg,
-        ),
-      );
+      updateMessage(messageId, { respondedAction: undefined });
     }
   };
 
@@ -87,7 +75,7 @@ const Chat = () => {
       side: "right",
       content: text,
     };
-    setMessages((prev) => [...prev, userMessage]);
+    addMessage(userMessage);
 
     try {
       // Fetch server response
@@ -101,7 +89,7 @@ const Chat = () => {
         proposedChange: response.message.proposedChange,
         conversationId: response.conversationId,
       };
-      setMessages((prev) => [...prev, botMessage]);
+      addMessage(botMessage);
     } catch (error) {
       console.error("Failed to send message:", error);
     }