fix(zmodem): 接收路径写背压；sftp 删除容忍 ENOENT；sysinfo 停止关流

- zmodem 接收：fs WriteStream 缓冲无上限，慢盘 + 快对端可把内存涨到接近
  文件大小。write() 返回 false 时暂停喂入 sentry 的 ssh 流、drain 恢复；
  touchActivity 只在写入被接受或 drain 恢复时计数，stall 看门狗不再把
  '堆在缓冲里'当进展；finalize/detach 解除悬挂暂停
- sftp 删除路径 ENOENT 一律视为已删成功（文件已被他人删除/重复删除/
  传输重跑后目标已消失），不再汇成假 delete failed；真错误仍报错
  （tests/sftp-timeout.mjs 新增 5 断言含防过度吞错用例）
- sysinfo：exec 回调里 state.stopped 分支先 close 到手的 stream

src/main/sysinfo.ts

@@ -157,7 +157,17 @@ function pollOnce(id: string, state: PollState): void {
 
   try {
     client.exec(COLLECT_CMD, (err: Error | undefined, stream) => {
-      if (state.stopped) return
+      if (state.stopped) {
+        // Polling stopped while this exec was in flight: the channel is already
+        // open, so simply dropping the stream would leak it for the rest of the
+        // ssh session. Close it (best effort — it may be dead already) and go.
+        try {
+          stream?.close()
+        } catch {
+          // best effort
+        }
+        return
+      }
       if (err || !stream) {
         handleError(id, state, err?.message || t('main.sysinfo.execFailed'))
         return