feat: implement chat persistence with MongoDB

- Add full chat persistence to database (conversations and messages)
- Implement MongoChatRepository with cursor-based pagination
- Add getConversations/getConversation endpoints in ChatController
- Save user and assistant messages in ChatService.processMessage()
- Track respondedAction (confirm/reject) on proposed event messages
- Load existing messages on chat screen mount
- Add addMessages() bulk action and chatMessageToMessageData() helper to ChatStore
- Add RespondedAction type and UpdateMessageDTO to shared types

apps/client/src/app/(tabs)/chat.tsx

@@ -1,18 +1,25 @@
-import { View, Text, TextInput, Pressable, KeyboardAvoidingView, Platform, Keyboard } from "react-native";
+import {
+  View,
+  Text,
+  TextInput,
+  Pressable,
+  KeyboardAvoidingView,
+  Platform,
+  Keyboard,
+} from "react-native";
 import currentTheme from "../../Themes";
-import { useState, useRef, useEffect } from "react";
+import React, { useState, useRef, useEffect } from "react";
 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 { useChatStore, chatMessageToMessageData, MessageData } from "../../stores";
 import { ProposedEventChange } from "@caldav/shared";
 import { ProposedEventCard } from "../../components/ProposedEventCard";
 
 // TODO: better shadows for everything
 // (maybe with extra library because of differences between android and ios)
 // TODO: max width for messages
-// TODO: create new messages
 
 type BubbleSide = "left" | "right";
 
@@ -30,14 +37,43 @@ type ChatInputProps = {
 };
 
 const Chat = () => {
-  const { messages, addMessage, updateMessage } = useChatStore();
-  const listRef = useRef<FlashList<MessageData>>(null);
+  const { messages, addMessage, addMessages, updateMessage } = useChatStore();
+  const listRef =
+    useRef<React.ComponentRef<typeof FlashList<MessageData>>>(null);
+  const [currentConversationId, setCurrentConversationId] = useState<
+    string | undefined
+  >();
 
   useEffect(() => {
-    const keyboardDidShow = Keyboard.addListener("keyboardDidShow", scrollToEnd);
+    const keyboardDidShow = Keyboard.addListener(
+      "keyboardDidShow",
+      scrollToEnd,
+    );
     return () => keyboardDidShow.remove();
   }, []);
 
+  // Load existing messages from database on mount
+  useEffect(() => {
+    const fetchMessages = async () => {
+      try {
+        const conversationSummaries = await ChatService.getConversations();
+        if (conversationSummaries.length > 0) {
+          const conversationId = conversationSummaries[0].id;
+          setCurrentConversationId(conversationId);
+
+          const serverMessages =
+            await ChatService.getConversation(conversationId);
+          const clientMessages = serverMessages.map(chatMessageToMessageData);
+          addMessages(clientMessages);
+          scrollToEnd();
+        }
+      } catch (error) {
+        console.error("Failed to load messages:", error);
+      }
+    };
+    fetchMessages();
+  }, []);
+
   const scrollToEnd = () => {
     setTimeout(() => {
       listRef.current?.scrollToEnd({ animated: true });
@@ -87,13 +123,22 @@ const Chat = () => {
       id: Date.now().toString(),
       side: "right",
       content: text,
+      conversationId: currentConversationId,
     };
     addMessage(userMessage);
     scrollToEnd();
 
     try {
-      // Fetch server response
-      const response = await ChatService.sendMessage({ content: text });
+      // Fetch server response (include conversationId for existing conversations)
+      const response = await ChatService.sendMessage({
+        content: text,
+        conversationId: currentConversationId,
+      });
+
+      // Track conversation ID for subsequent messages
+      if (!currentConversationId) {
+        setCurrentConversationId(response.conversationId);
+      }
 
       // Show bot response
       const botMessage: MessageData = {