Port OnlineCTR to CrossPlatform (Native Linux Support)

[Diassisfilho]

This PR restructures the OnlineCTR module and introduces native Linux support for both the Client and Server.

Key Changes

1. Moved mods/Windows to mods/CrossPlatform: Since the code now compiles natively on both Windows and Linux, the folder has been renamed to reflect its platform-agnostic nature.
2. Process Detection: The Client now falls back to scanning /proc (including comm and cmdline) to accurately detect DuckStation and AppImage instances on Linux.
3. Shared Memory: On Linux, the Client utilizes shm_open and mmap instead of Win32 MapViewOfFile to read DuckStation's exported memory.
4. Cross-Platform Compatibility: Replaced non-standard functions (like strcpy_s) with safe macros (STRCPY_SAFE), abstracted Sleep/usleep, and fixed _Static_assert compilation under C++ (g++).
5. CMake Build System: Added a CMakeLists.txt to the Client and updated the Server's CMakeLists.txt to build ENet directly from its source submodule.
6. Documentation: Added README-Linux.md outlining the build and run instructions for Linux users.