fix(chat): 撤回消息时回退未读角标，避免当成新消息

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/chat/ChatShell.tsx

@@ -175,6 +175,7 @@ export default function ChatShell({
     const off = realtime.on<RtChatUnreadData>(RT_CHAT_UNREAD, (data) => {
       if (!data?.room_id) return;
       if (activeRoomId === data.room_id) return;
+      const delta = typeof data.delta === "number" ? data.delta : 1;
       setRooms((prev) => {
         const idx = prev.findIndex((r) => r.id === data.room_id);
         if (idx < 0) {
@@ -183,9 +184,17 @@ export default function ChatShell({
         }
         const next = [...prev];
         const cur = next[idx];
+        // 撤回回退：只改角标，预览由 chat:message_recalled 负责，避免覆盖「已撤回」
+        if (delta < 0) {
+          next[idx] = {
+            ...cur,
+            unread_count: Math.max(0, (cur.unread_count || 0) + delta),
+          };
+          return next;
+        }
         next[idx] = {
           ...cur,
-          unread_count: (cur.unread_count || 0) + (data.delta || 1),
+          unread_count: (cur.unread_count || 0) + delta,
           last_message_id: data.message_id,
           last_message: {
             id: data.message_id,