CMake: silence CMP0167 from MRViewer Boost::locale lookup

[Fedr]

Summary

Switch the find_package(Boost ... locale) call in source/MRViewer/CMakeLists.txt to CONFIG mode on non-Emscripten builds, mirroring the pattern already used in MRMesh/CMakeLists.txt and MRPch/CMakeLists.txt.

This silences the CMP0167 deprecation warning ("The FindBoost module is removed") emitted by CMake 3.30+ when the legacy FindBoost module is invoked indirectly via vcpkg's Boost wrapper.

Why

CMake 3.30 removed the bundled FindBoost module in favor of Boost's own BoostConfig.cmake. Calling find_package(Boost ...) without CONFIG falls into the legacy module path, triggering:

CMake Warning (dev) at C:/vcpkg/installed/.../share/boost/vcpkg-cmake-wrapper.cmake:3 (_find_package):
  Policy CMP0167 is not set: The FindBoost module is removed.  Run "cmake
  --help-policy CMP0167" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.
Call Stack (most recent call first):
  C:/vcpkg/scripts/buildsystems/vcpkg.cmake:813 (include)
  source/MRViewer/CMakeLists.txt:110 (find_package)

vcpkg already ships BoostConfig.cmake (its wrapper just forwards to it), so switching to CONFIG mode is a no-op functionally — the same Boost is found, only the legacy code path is avoided.

The Emscripten branch keeps module mode because vcpkg's Emscripten Boost port doesn't install CMake config files; that branch additionally sets CMP0167 OLD so the warning stays suppressed there too.

Test plan

• CMP0167 warning gone from the windows-vs2019-meshlib configure log (the job that originated the warning).
• Windows MSVC 2019 / MSVC 2022 build & test jobs pass.
• Linux x64 (GCC 12/13/14, Clang 20) and arm64 (GCC 11, Clang 20, ubuntu22/24) build & test jobs pass.
• macOS arm64 Debug + Release build & test jobs pass.
• Emscripten Singlethreaded / Multithreaded / Multithreaded-64Bit configure, build & runtime tests pass — Boost::locale linkage unchanged on WASM.