fix(server): Prevent crash from dangling pointers in file dialog map

Fixed two bugs when closing ScreenSpyDlg with file transfer dialogs:

1. Access violation (0xC0000005): CDlgFileSend self-destructs via
   PostNcDestroy (delete this) when closed, leaving dangling pointers
   in m_FileRecvDlgs map.

2. Double-free: Original code called DestroyWindow() then delete,
   but DestroyWindow already triggers delete this via PostNcDestroy.

Solution:
- Store {HWND, pointer} pairs instead of raw pointers
- Check HWND validity with IsWindow() before accessing pointer
- Use SendMessage(WM_CLOSE) to let dialog self-destruct safely
- Always erase map entries to prevent accumulation of invalid data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/2015Remote/ScreenSpyDlg.h

@@ -198,7 +198,8 @@ public:
 
     // 文件接收进度对话框（用于 Linux Ctrl+C -> 服务端 Ctrl+V）
     // 按 transferID 管理多个并发传输
-    std::map<uint64_t, class CDlgFileSend*> m_FileRecvDlgs;
+    // 存储 {HWND, 指针} 对，HWND 用于安全检查（指针可能变成野指针）
+    std::map<uint64_t, std::pair<HWND, class CDlgFileSend*>> m_FileRecvDlgs;
 
     void SaveSnapshot(void);
     // 对话框数据