feat(web): D4 会话未读徽标 — 打开即已读 + 消费 session_read

- SessionMeta.unreadCount（可选，兼容旧后端）+ api/sessions.markSessionRead
- lib/sessionRead.ts：会话历史落地后再推游标（best-effort，失败静默，下次 GET /sessions 收敛）+ 帧归属判定（ws.ts 因此留在 500 行内）
- store.clearUnread 无变化时返回同一 state，避免多余重渲染；Nav 徽标沿用 Miuix token（mu-radius-pill/mu-err/mu-rise-in）
- 逻辑测试 35 → 54 项，含变异校验（改成 clearUnread(activeSessionId) 即 FAIL）

frontend/web/src/store/useAppStore.ts

@@ -63,6 +63,11 @@ export interface AppStore {
 
   /** Replace the session metadata list. */
   setSessions: (list: SessionMeta[]) => void;
+  /** Zero one session's unread badge locally (Android 协同 B3/D4). Called when the
+   *  user opens the session and its page has loaded, and on the `session_read`
+   *  frame another end broadcasts — a no-op for an already-read or unknown
+   *  session, so neither path can invent a row or re-render the list needlessly. */
+  clearUnread: (sessionId: string) => 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). */
@@ -144,6 +149,14 @@ export const useAppStore = create<AppStore>()(
           sessions: Object.fromEntries(list.map((s) => [s.id, s])),
         }),
       setActiveSession: (id) => set({ activeSessionId: id, messages: [] }),
+      clearUnread: (sessionId) =>
+        set((s) => {
+          const meta = s.sessions[sessionId];
+          if (!meta?.unreadCount) return s;
+          return {
+            sessions: { ...s.sessions, [sessionId]: { ...meta, unreadCount: 0 } },
+          };
+        }),
       setMessages: (list) => set({ messages: list }),
       prependMessages: (list) =>
         set((s) => {