Feature: screen preview thumbnail on host double-click

Server sends COMMAND_SCREEN_PREVIEW_REQ when user double-clicks an
active (non-Locked/Inactive) host that advertises CLIENT_CAP_SCREEN_PREVIEW.
Client BitBlts primary screen, encodes to JPEG via GDI+ and replies. The
existing STATIC tooltip is replaced with a self-drawn CPreviewTipWnd
showing the thumbnail above the host info text, with wide-character
rendering so the popup also works on non-Chinese servers.

- Quality tiers reuse QualityProfile pattern: PreviewProfile + 6 levels
  driven by GetTargetQualityLevel (FRP-aware), with 4K/ultrawide auto
  upscale on Ultra/High tiers up to min(screenWidth/4, 1280).
- Client limits to 1 in-flight capture via atomic counter to defend
  against flood/DoS; Send2Server is already mutex-serialized.
- Server validates responses by reqId only (single in-flight tip);
  4s arrival timeout marks "preview unavailable" without blocking the
  text fallback path.

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

server/2015Remote/2015RemoteDlg.h

@@ -230,6 +230,10 @@ public:
     VOID SendAllCommand(PBYTE  szBuffer, ULONG ulLength);
     // 显示用户上线信息
     CWnd* m_pFloatingTip = nullptr;
+    // 屏幕预览：m_pFloatingTip 实际是 CPreviewTipWnd 时这里有同一指针的有类型副本，
+    // 用于在收到 JPEG 后调用 SetImageFromJpeg；DeletePopupWindow 释放时一并置空。
+    class CPreviewTipWnd* m_pPreviewTip = nullptr;
+    WORD m_PreviewReqId = 0;            // 当前期待的预览响应序号；0 = 无待响应
     // 记录 clientID（心跳更新）
     std::set<uint64_t> m_DirtyClients;
     // 待处理的上线/下线事件（批量更新减少闪烁）
@@ -434,6 +438,12 @@ public:
     afx_msg void OnWhatIsThis();
     afx_msg void OnOnlineAuthorize();
     void OnListClick(NMHDR* pNMHDR, LRESULT* pResult);
+    // 屏幕预览：依 ctx 最近 RTT + 屏幕分辨率挑参数；4K/超宽屏在 LAN 档自适应放大
+    void ChooseScreenPreviewParams(context* ctx, WORD& maxWidth, BYTE& jpegQuality) const;
+    // 发起预览请求；reqId 应与 m_PreviewReqId 同步
+    void SendScreenPreviewRequest(context* ctx, WORD reqId, WORD maxWidth, BYTE jpegQuality);
+    // 收到 TOKEN_SCREEN_PREVIEW_RSP（在主线程处理）
+    afx_msg LRESULT OnPreviewResponse(WPARAM wParam, LPARAM lParam);
     afx_msg void OnOnlineUnauthorize();
     afx_msg void OnToolRequestAuth();
     afx_msg LRESULT OnPasswordCheck(WPARAM wParam, LPARAM lParam);