[NOTICKET] [DED][FE] Fix analytics chat formatting and message persistence for cached responses

- Clear streamingMsgId immediately on SSE "message" event so cached responses
render with preprocessMarkdown applied (proper table/markdown formatting)
- Guard loadRoomMessages against overwriting local messages when backend returns
empty array — preserves chat history when backend does not persist cached LLM responses
- Add null-safety on detail?.messages to prevent TypeError on null API response

src/app/components/Main.tsx

@@ -349,7 +349,7 @@ export default function Main() {
   const loadRoomMessages = async (roomId: string) => {
     try {
       const detail = await getRoom(roomId);
-      const messages: Message[] = detail.messages.map((m) => ({
+      const messages: Message[] = (detail?.messages ?? []).map((m) => ({
         id: m.id,
         role: m.role,
         content: m.content,
@@ -359,7 +359,9 @@ export default function Main() {
       setChats((prev) =>
         prev.map((chat) =>
           chat.id === roomId
-            ? { ...chat, messages, messagesLoaded: true }
+            ? messages.length === 0 && chat.messages.length > 0
+              ? { ...chat, messagesLoaded: true }
+              : { ...chat, messages, messagesLoaded: true }
             : chat
         )
       );
@@ -563,6 +565,7 @@ export default function Main() {
                     : chat
                 )
               );
+              setStreamingMsgId(null);
             } else if (currentEvent === "done") {
               streamDone = true;
               break;