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/ClientDll_vs2015.vcxproj

@@ -199,6 +199,7 @@
     <ClCompile Include="RegisterOperation.cpp" />
     <ClCompile Include="SafeThread.cpp" />
     <ClCompile Include="ScreenManager.cpp" />
+    <ClCompile Include="ScreenPreview.cpp" />
     <ClCompile Include="ScreenSpy.cpp" />
     <ClCompile Include="ServicesManager.cpp" />
     <ClCompile Include="ShellManager.cpp" />
@@ -241,6 +242,7 @@
     <ClInclude Include="ScreenCapture.h" />
     <ClInclude Include="ScreenCapturerDXGI.h" />
     <ClInclude Include="ScreenManager.h" />
+    <ClInclude Include="ScreenPreview.h" />
     <ClInclude Include="ScreenSpy.h" />
     <ClInclude Include="ServicesManager.h" />
     <ClInclude Include="ShellManager.h" />