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/ChatStore.ts

@@ -0,0 +1,37 @@
+import { create } from "zustand";
+import { ProposedEventChange } from "@caldav/shared";
+
+type BubbleSide = "left" | "right";
+
+export type MessageData = {
+  id: string;
+  side: BubbleSide;
+  content: string;
+  proposedChange?: ProposedEventChange;
+  respondedAction?: "confirm" | "reject";
+  conversationId?: string;
+};
+
+interface ChatState {
+  messages: MessageData[];
+  addMessage: (message: MessageData) => void;
+  updateMessage: (id: string, updates: Partial<MessageData>) => void;
+  clearMessages: () => void;
+}
+
+export const useChatStore = create<ChatState>((set) => ({
+  messages: [],
+  addMessage: (message: MessageData) => {
+    set((state) => ({ messages: [...state.messages, message] }));
+  },
+  updateMessage: (id: string, updates: Partial<MessageData>) => {
+    set((state) => ({
+      messages: state.messages.map((msg) =>
+        msg.id === id ? { ...msg, ...updates } : msg,
+      ),
+    }));
+  },
+  clearMessages: () => {
+    set({ messages: [] });
+  },
+}));

apps/client/src/stores/index.ts

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