feat(web): 前端接会话持久化——拉历史热分页 + 侧栏会话列表

- store: 消息真源移到后端（sessions元数据map + activeSessionId，partialize 移除 messages 改存会话元数据，version:2 migrate 清旧数据）
- api: fetchSessions/createSubSession/fetchMessages(热分页before游标)/deleteSession + MessageDto→toChatMessage
- Nav: 侧栏「会话」区——主会话置顶 + 次级列表 + ＋新建会话（Miuix）
- 新增 HistoryList: 顶部「加载更早」向上分页 + 保守虚拟化(>200只渲染最近200)
- useChat: send 用 activeSessionId；App: token 后 fetchSessions→激活主会话→拉近50条
- Docs/api-client: v0.10 补 WS sessionId + history 回放事件契约

frontend/web/src/modes/ChatShell.tsx

@@ -3,7 +3,7 @@ import { useAppStore } from "../store/useAppStore";
 import { useModeStore } from "../store/useModeStore";
 import { useSendChat } from "../lib/useChat";
 import { sendUp } from "../lib/ws";
-import { SplitChatBubble } from "../components/chat";
+import { HistoryList } from "../components/messages/HistoryList";
 import { ChatInput } from "../components/ChatInput";
 import { EmptyState } from "../components/primitives";
 import { Icon } from "../components/Icon";
@@ -49,9 +49,13 @@ export function ChatShell(): React.ReactNode {
     return undefined;
   }, [messages]);
 
+  // Scroll to the newest message only when a *new* message reaches the tail
+  // (append/session-switch), not when the user hot-pages older history into the
+  // list — otherwise "load earlier" would yank the viewport to the bottom.
+  const lastMessageId = messages[messages.length - 1]?.id;
   useEffect(() => {
     bottomRef.current?.scrollIntoView({ behavior: "smooth", block: "end" });
-  }, [messages]);
+  }, [lastMessageId]);
 
   return (
     <div className="flex h-full flex-col">
@@ -77,17 +81,7 @@ export function ChatShell(): React.ReactNode {
                 hint="模式只改变消息框的呈现方式，会话上下文与记忆保持不变"
               />
             ) : (
-              messages.map((m) => (
-                <SplitChatBubble
-                  key={m.id}
-                  message={m}
-                  active={
-                    m.id === lastAssistantId &&
-                    m.role === "assistant" &&
-                    (m.streaming ?? false)
-                  }
-                />
-              ))
+              <HistoryList lastAssistantId={lastAssistantId} />
             )}
             <div ref={bottomRef} />
           </div>