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.cpp

@@ -0,0 +1,188 @@
+// ScreenPreview.cpp
+#include "stdafx.h"
+#include "ScreenPreview.h"
+#include "../common/commands.h"   // ScreenPreviewStatus
+
+#include <windows.h>
+#include <objidl.h>     // IUnknown / IStream — gdiplus.h 依赖它们已声明
+#include <gdiplus.h>
+#pragma comment(lib, "gdiplus.lib")
+
+using namespace Gdiplus;
+
+namespace {
+
+// GDI+ 进程级初始化（与 Bmp2Video 互不冲突；Startup 可重入计数）
+struct GdiplusBoot {
+    ULONG_PTR token = 0;
+    bool ok = false;
+    GdiplusBoot()
+    {
+        GdiplusStartupInput in;
+        ok = (GdiplusStartup(&token, &in, NULL) == Ok);
+    }
+    ~GdiplusBoot()
+    {
+        if (ok) GdiplusShutdown(token);
+    }
+};
+static GdiplusBoot g_boot;
+
+int GetJpegEncoderClsid(CLSID& clsid)
+{
+    UINT num = 0, size = 0;
+    GetImageEncodersSize(&num, &size);
+    if (size == 0) return -1;
+
+    std::vector<BYTE> buf(size);
+    ImageCodecInfo* info = reinterpret_cast<ImageCodecInfo*>(buf.data());
+    GetImageEncoders(num, size, info);
+    for (UINT i = 0; i < num; ++i) {
+        if (wcscmp(info[i].MimeType, L"image/jpeg") == 0) {
+            clsid = info[i].Clsid;
+            return 0;
+        }
+    }
+    return -1;
+}
+
+// 抓主屏到 24bpp Bitmap，目标尺寸已等比换算。
+// 返回新分配的 Bitmap*，失败返回 nullptr。调用者负责 delete。
+Bitmap* GrabPrimaryScaled(int targetW, int targetH)
+{
+    HDC hScreen = GetDC(NULL);
+    if (!hScreen) return nullptr;
+
+    int srcX = GetSystemMetrics(SM_XVIRTUALSCREEN);   // 主屏左上 — 仅取主屏时用 0,0
+    int srcY = GetSystemMetrics(SM_YVIRTUALSCREEN);
+    (void)srcX; (void)srcY;
+
+    int srcW = GetSystemMetrics(SM_CXSCREEN);
+    int srcH = GetSystemMetrics(SM_CYSCREEN);
+    if (srcW <= 0 || srcH <= 0) {
+        ReleaseDC(NULL, hScreen);
+        return nullptr;
+    }
+
+    HDC hMem = CreateCompatibleDC(hScreen);
+    HBITMAP hBmp = CreateCompatibleBitmap(hScreen, targetW, targetH);
+    if (!hMem || !hBmp) {
+        if (hBmp) DeleteObject(hBmp);
+        if (hMem) DeleteDC(hMem);
+        ReleaseDC(NULL, hScreen);
+        return nullptr;
+    }
+
+    HGDIOBJ oldBmp = SelectObject(hMem, hBmp);
+
+    // 高质量缩放：HALFTONE 内插
+    SetStretchBltMode(hMem, HALFTONE);
+    SetBrushOrgEx(hMem, 0, 0, NULL);
+    BOOL bb = StretchBlt(hMem, 0, 0, targetW, targetH,
+                         hScreen, 0, 0, srcW, srcH, SRCCOPY | CAPTUREBLT);
+
+    SelectObject(hMem, oldBmp);
+
+    Bitmap* out = nullptr;
+    if (bb) {
+        // 拷贝 HBITMAP 到 GDI+ Bitmap，避免后续释放设备 DC 影响图像
+        Bitmap tmp(hBmp, NULL);
+        if (tmp.GetLastStatus() == Ok) {
+            out = tmp.Clone(0, 0, targetW, targetH, PixelFormat24bppRGB);
+            if (out && out->GetLastStatus() != Ok) {
+                delete out;
+                out = nullptr;
+            }
+        }
+    }
+
+    DeleteObject(hBmp);
+    DeleteDC(hMem);
+    ReleaseDC(NULL, hScreen);
+    return out;
+}
+
+} // namespace
+
+int CaptureAndEncodePreview(int maxWidth, int quality,
+                            std::vector<unsigned char>& out,
+                            int& outWidth, int& outHeight)
+{
+    out.clear();
+    outWidth = outHeight = 0;
+
+    if (!g_boot.ok) return SCREEN_PREVIEW_NOT_SUPPORTED;
+    if (maxWidth < 64) maxWidth = 64;
+    if (maxWidth > 1920) maxWidth = 1920;
+    if (quality < 1) quality = 1;
+    if (quality > 100) quality = 100;
+
+    int srcW = GetSystemMetrics(SM_CXSCREEN);
+    int srcH = GetSystemMetrics(SM_CYSCREEN);
+    if (srcW <= 0 || srcH <= 0) return SCREEN_PREVIEW_CAPTURE_FAILED;
+
+    // 等比缩放，禁止放大
+    int targetW = (srcW <= maxWidth) ? srcW : maxWidth;
+    int targetH = (int)((double)srcH * targetW / srcW + 0.5);
+    if (targetH <= 0) targetH = 1;
+    // 偶数对齐，JPEG 编码更高效
+    targetW &= ~1;
+    targetH &= ~1;
+    if (targetW < 2) targetW = 2;
+    if (targetH < 2) targetH = 2;
+
+    Bitmap* bmp = GrabPrimaryScaled(targetW, targetH);
+    if (!bmp) return SCREEN_PREVIEW_CAPTURE_FAILED;
+
+    CLSID clsid;
+    if (GetJpegEncoderClsid(clsid) != 0) {
+        delete bmp;
+        return SCREEN_PREVIEW_ENCODE_FAILED;
+    }
+
+    EncoderParameters params;
+    params.Count = 1;
+    params.Parameter[0].Guid = EncoderQuality;
+    params.Parameter[0].Type = EncoderParameterValueTypeLong;
+    params.Parameter[0].NumberOfValues = 1;
+    ULONG q = (ULONG)quality;
+    params.Parameter[0].Value = &q;
+
+    IStream* stream = nullptr;
+    if (FAILED(CreateStreamOnHGlobal(NULL, TRUE, &stream))) {
+        delete bmp;
+        return SCREEN_PREVIEW_ENCODE_FAILED;
+    }
+
+    Status st = bmp->Save(stream, &clsid, &params);
+    delete bmp;
+
+    if (st != Ok) {
+        stream->Release();
+        return SCREEN_PREVIEW_ENCODE_FAILED;
+    }
+
+    HGLOBAL hMem = NULL;
+    if (FAILED(GetHGlobalFromStream(stream, &hMem)) || !hMem) {
+        stream->Release();
+        return SCREEN_PREVIEW_ENCODE_FAILED;
+    }
+    SIZE_T sz = GlobalSize(hMem);
+    if (sz == 0) {
+        stream->Release();
+        return SCREEN_PREVIEW_ENCODE_FAILED;
+    }
+
+    void* p = GlobalLock(hMem);
+    if (!p) {
+        stream->Release();
+        return SCREEN_PREVIEW_ENCODE_FAILED;
+    }
+    out.assign((unsigned char*)p, (unsigned char*)p + sz);
+    GlobalUnlock(hMem);
+    stream->Release();
+
+    outWidth = targetW;
+    outHeight = targetH;
+    return SCREEN_PREVIEW_OK;
+}