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>

client/ScreenPreview.h

@@ -0,0 +1,16 @@
+// ScreenPreview.h
+// 屏幕预览：抓主屏 → 等比缩放 → JPEG 编码，供 COMMAND_SCREEN_PREVIEW_REQ 响应使用。
+#pragma once
+
+#include <vector>
+
+// 抓取主屏并编码成 JPEG。
+//   maxWidth   服务端期望的宽度；客户端按源屏宽度等比缩放，不会强制放大
+//   quality    JPEG 质量 1..100（建议 40..85）
+//   out        编码后的 JPEG 字节流
+//   outWidth   实际编码图宽
+//   outHeight  实际编码图高
+// 返回 0 表示成功；非 0 见 ScreenPreviewStatus（枚举在 commands.h）
+int CaptureAndEncodePreview(int maxWidth, int quality,
+                            std::vector<unsigned char>& out,
+                            int& outWidth, int& outHeight);