feat: 多频道任务队列总览卡片

- download_stat 新增 _task_queue / _completed_chats 与三个辅助函数
- 切频道前自动把上一个频道的最终进度快照进 completed_chats
- save_and_restart(_multi) 启动任务时设置队列，供前端渲染总览
- 前端 banner 改为"任务队列"多卡片列表（✅已完成 / 🚀当前 / ⏳排队中），
  含进度条、跳过明细；兼容单频道场景

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

module/download_stat.py

@@ -43,6 +43,12 @@ _task_progress: dict = {
     "last_update": 0,
 }
 
+# 多频道任务队列与已完成快照（独立于 _task_progress，不被单频道 reset 影响）
+# _task_queue: 任务开始时由 API 设置，[{chat_id, chat_title}, ...]
+# _completed_chats: 每切换一个频道前把上一个的最终进度快照到这里
+_task_queue: list = []
+_completed_chats: list = []
+
 
 def get_download_result() -> dict:
     """get global download result"""
@@ -62,7 +68,7 @@ def get_download_state() -> DownloadState:
 def get_task_progress() -> dict:
     """get task progress with auto-detection of checking state"""
     progress = _task_progress.copy()
-    
+
     # Auto-detect if still checking based on last_update time
     # If last update was within 3 seconds, consider it still active
     if progress["current_chat"] and progress["last_update"] > 0:
@@ -73,10 +79,57 @@ def get_task_progress() -> dict:
             progress["is_checking"] = True
         elif progress["skipped_files"] > 0 or progress["checked_messages"] > 0:
             progress["is_checking"] = False
-    
+
+    # 附加多频道任务总览，供前端渲染队列卡片
+    progress["task_queue"] = list(_task_queue)
+    progress["completed_chats"] = list(_completed_chats)
     return progress
 
 
+def snapshot_current_chat():
+    """把当前 _task_progress 的核心字段快照进 _completed_chats。current_chat 为空时 no-op。
+    在多频道串行下载时，每个 download_chat_task 开始前调用，保留上一个频道的最终进度。
+    """
+    global _completed_chats
+    chat_id = _task_progress.get("current_chat", "")
+    if not chat_id:
+        return
+    qual = _task_progress.get("qualified_files", 0) or 0
+    est = _task_progress.get("estimated_total", 0) or 0
+    existing = _task_progress.get("existing_skipped", 0) or 0
+    raw_total = est or qual
+    real_total = max(0, raw_total - existing)
+    _completed_chats.append({
+        "chat_id": chat_id,
+        "chat_title": _task_progress.get("current_chat_title", "") or chat_id,
+        "done": _task_progress.get("completed_files", 0) or 0,
+        "total": real_total,
+        "skip": _task_progress.get("skipped_files", 0) or 0,
+        "existing_skip": existing,
+        "failed": _task_progress.get("failed_files", 0) or 0,
+    })
+
+
+def set_task_queue(items: list):
+    """设置本次任务的完整频道队列。items 每项 {chat_id, chat_title}。"""
+    global _task_queue
+    _task_queue = []
+    for it in (items or []):
+        cid = str(it.get("chat_id", "") or "").strip()
+        if not cid:
+            continue
+        _task_queue.append({
+            "chat_id": cid,
+            "chat_title": it.get("chat_title", "") or cid,
+        })
+
+
+def clear_completed_chats():
+    """新任务启动时清空已完成列表。"""
+    global _completed_chats
+    _completed_chats = []
+
+
 def update_task_progress(
     current_chat: str = None,
     current_chat_title: str = None,