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/context.h

@@ -68,4 +68,11 @@ public:
         if (caps.IsEmpty()) return false;
         return (strtoul(caps.GetString(), nullptr, 16) & CLIENT_CAP_UTF8) != 0;
     }
+
+    // 检查客户端是否支持屏幕预览（双击主机时拉缩略图）。
+    bool SupportsScreenPreview() const {
+        CString caps = GetClientData(ONLINELIST_CAPABILITIES);
+        if (caps.IsEmpty()) return false;
+        return (strtoul(caps.GetString(), nullptr, 16) & CLIENT_CAP_SCREEN_PREVIEW) != 0;
+    }
 };