fix: stabilize legal chat streaming

frontend/src/app/chat/page.tsx

@@ -655,18 +655,16 @@ function normalizeMessagesForDisplay(items: ChatMessage[], lastQuestion: string,
 }
 
 function mergeAssistantContent(previous: string, incoming: string) {
-  const trimmedPrevious = previous.trim();
-  const trimmedIncoming = incoming.trim();
-  if (!trimmedIncoming) {
+  if (!incoming) {
     return previous;
   }
-  if (!trimmedPrevious) {
-    return trimmedIncoming;
+  if (!previous) {
+    return incoming.trimStart();
   }
-  if (trimmedIncoming.startsWith(trimmedPrevious)) {
-    return trimmedIncoming;
+  if (incoming.startsWith(previous)) {
+    return incoming;
   }
-  return `${trimmedPrevious}\n\n${trimmedIncoming}`;
+  return `${previous}${incoming}`;
 }
 
 function dedupeMessages(items: ChatMessage[]): ChatMessage[] {

frontend/src/services/chat.ts

@@ -8,7 +8,7 @@ import {
 } from "@/services/http";
 import type { ChatSourceItem, ChatStreamEvent, ConversationDetail, ConversationListItem, PageResponse, ReviewChatContext } from "@/types/api";
 
-const STREAM_TIMEOUT_MS = 30000;
+const STREAM_TIMEOUT_MS = 180000;
 
 export type StreamChatParams = {
   message: string;