feat: IM式跨端实时同步——message_persisted 广播 + 前端按id去重追加

- history.Store 新增 AppendFull（返回含 id/createdAt 的完整消息），Append 签名不变
- session_orchestrator 加可空 OnPersisted；persistUserMessages/persistAssistantReply 落库后回调广播
- server.go newOrchestrator 接 OnPersisted → hub.broadcast message_persisted（thinking 剥除）
- 前端 ws.ts 加 message_persisted case（仅 activeSessionId 匹配才处理）；appendOrUpdateMessage 升级为 id 精确upsert+内容/时间归一
- 实测：双 WS，A stream 发消息，B 免刷新收到 user+assistant 各1条（count=2 PASS）
- 红线: 身份仍靠 actor 元数据，不靠提示词；用户/助手消息、本端/异端统一路径

frontend/web/src/store/useAppStore.ts

@@ -61,7 +61,9 @@ export interface AppStore {
   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). */
+  /** Append a message, or merge by id if it already exists. On message_persisted
+   *  it also adopts a same-role local optimistic bubble (rewriting its uid to the
+   *  backend id) so a message never renders twice (design/16 §4.3 dedup). */
   appendOrUpdateMessage: (msg: ChatMessage) => void;
 
   addUserMessage: (text: string, mode?: Mode, image?: string) => void;
@@ -135,11 +137,48 @@ export const useAppStore = create<AppStore>()(
         }),
       appendOrUpdateMessage: (msg) =>
         set((s) => {
+          // Exact id match → merge. Keeps local streaming/tools/thinking, takes
+          // the backend's content/mode/mood as truth.
           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 };
+          if (idx !== -1) {
+            const messages = s.messages.slice();
+            messages[idx] = { ...messages[idx], ...msg };
+            return { messages };
+          }
+          // No exact id: adopt a local optimistic bubble (no sessionId) of the
+          // same role, so a message the local browser already rendered
+          // optimistically is never duplicated when message_persisted arrives
+          // (design/16 §4.3 dedup by id). Prefer a content match, else the
+          // nearest by createdAt; scan newest-first.
+          let best = -1;
+          let bestDist = Number.MAX_SAFE_INTEGER;
+          for (let i = s.messages.length - 1; i >= 0; i--) {
+            const m = s.messages[i];
+            if (m.sessionId || m.role !== msg.role || m.id === msg.id) continue;
+            const dist = m.content === msg.content ? 0 : Math.abs(m.createdAt - msg.createdAt);
+            if (dist < bestDist) {
+              bestDist = dist;
+              best = i;
+            }
+          }
+          if (best !== -1) {
+            const m = s.messages[best];
+            const messages = s.messages.slice();
+            messages[best] = {
+              ...m,
+              id: msg.id,
+              sessionId: msg.sessionId ?? m.sessionId,
+              content: msg.content || m.content,
+              mode: msg.mode ?? m.mode,
+              mood: msg.mood ?? m.mood,
+              image: msg.image ?? m.image,
+              createdAt: msg.createdAt ?? m.createdAt,
+            };
+            return { messages };
+          }
+          // No local bubble (e.g. a remote browser that never optimistically
+          // rendered this message): append.
+          return { messages: [...s.messages, msg] };
         }),
 
       addUserMessage: (text, mode, image) =>