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/store/useAppStore.ts

@@ -8,6 +8,7 @@ import type {
   FlowGraph,
   Mode,
   PersonaRecord,
+  SessionMeta,
   ToolUsed,
   WsStatus,
 } from "../lib/types";
@@ -20,8 +21,15 @@ interface ChatContext {
 }
 
 export interface AppStore {
-  sessionId: string;
+  /** Session metadata (main + secondary) keyed by session id. The message truth
+   *  lives server-side, so this is a small list — safe to persist. */
+  sessions: Record<string, SessionMeta>;
+  /** The session the UI is currently focused on (design/16 §2.1). "" until the
+   *  session list is loaded. Messages in `messages` always belong to this id. */
+  activeSessionId: string;
   wsStatus: WsStatus;
+  /** Messages of the *active* session, in display order (oldest first). Reset on
+   *  session switch and refilled from the backend (design/16 §4/§5). */
   messages: ChatMessage[];
   mood: string | null;
   context: ChatContext;
@@ -41,11 +49,21 @@ export interface AppStore {
   currentPersona: PersonaRecord | null;
 
   setWsStatus: (status: WsStatus) => void;
-  setSessionId: (id: string) => void;
   setCurrentPersona: (p: PersonaRecord | null) => void;
   setContext: (patch: Partial<ChatContext>) => void;
   setMood: (mood: string | null) => void;
 
+  /** Replace the session metadata list. */
+  setSessions: (list: SessionMeta[]) => void;
+  /** Focus a session and clear its message cache (refilled by the caller). */
+  setActiveSession: (id: string) => void;
+  /** Replace the active session's message list (initial page on switch). */
+  setMessages: (list: ChatMessage[]) => void;
+  /** Prepend an older page (chronological) onto the active session's messages. */
+  prependMessages: (list: ChatMessage[]) => void;
+  /** Append a new message, or merge by id if it already exists (WS/dedup). */
+  appendOrUpdateMessage: (msg: ChatMessage) => void;
+
   addUserMessage: (text: string, mode?: Mode, image?: string) => void;
   beginAssistant: (mode?: Mode) => string;
   appendChunk: (delta: string) => void;
@@ -77,7 +95,8 @@ export interface AppStore {
 export const useAppStore = create<AppStore>()(
   persist(
     (set, get) => ({
-      sessionId: "web",
+      sessions: {},
+      activeSessionId: "",
       wsStatus: "idle",
       messages: [],
       mood: null,
@@ -97,12 +116,32 @@ export const useAppStore = create<AppStore>()(
       taskLogs: {},
 
       setWsStatus: (status) => set({ wsStatus: status }),
-      setSessionId: (id) => set({ sessionId: id }),
       setCurrentPersona: (p) => set({ currentPersona: p }),
       setContext: (patch) =>
         set((s) => ({ context: { ...s.context, ...patch } })),
       setMood: (mood) => set({ mood }),
 
+      setSessions: (list) =>
+        set({
+          sessions: Object.fromEntries(list.map((s) => [s.id, s])),
+        }),
+      setActiveSession: (id) => set({ activeSessionId: id, messages: [] }),
+      setMessages: (list) => set({ messages: list }),
+      prependMessages: (list) =>
+        set((s) => {
+          const seen = new Set(s.messages.map((m) => m.id));
+          const older = list.filter((m) => !seen.has(m.id));
+          return { messages: [...older, ...s.messages] };
+        }),
+      appendOrUpdateMessage: (msg) =>
+        set((s) => {
+          const idx = s.messages.findIndex((m) => m.id === msg.id);
+          if (idx === -1) return { messages: [...s.messages, msg] };
+          const messages = s.messages.slice();
+          messages[idx] = { ...messages[idx], ...msg };
+          return { messages };
+        }),
+
       addUserMessage: (text, mode, image) =>
         set((s) => ({
           messages: [
@@ -249,16 +288,32 @@ export const useAppStore = create<AppStore>()(
     }),
     {
       name: "cyrene.app",
-      version: 1,
+      version: 2,
+      // Upgrade from v1 (which persisted a full `messages` array + `sessionId`):
+      // drop those — the backend is now the message truth — and carry forward the
+      // still-meaningful bits (persona avatar, mood, context, backthink).
+      migrate: (persisted) => {
+        const p = (persisted ?? {}) as Partial<AppStore>;
+        return {
+          activeSessionId: "",
+          sessions: {},
+          mood: p.mood ?? null,
+          context: p.context ?? {},
+          backthink: p.backthink ?? [],
+          currentPersona: p.currentPersona ?? null,
+        };
+      },
       partialize: (s) => ({
-        sessionId: s.sessionId,
-        messages: s.messages,
+        // messages deliberately excluded: the backend is the message truth
+        // (design/16 §1), so we never persist the whole chat into localStorage.
+        // We keep the (small) session metadata + the focused session id so a
+        // refresh lands on the same conversation, plus the resolved persona for
+        // the avatar/background.
+        activeSessionId: s.activeSessionId,
+        sessions: s.sessions,
         mood: s.mood,
         context: s.context,
         backthink: s.backthink,
-        // Persist the resolved persona so a refresh keeps showing THIS persona's
-        // own avatar/background — never falling back to a neutral placeholder or
-        // (worse) another role's image because the mount-time fetch missed.
         currentPersona: s.currentPersona,
       }),
     }