feat: add ASIO signal handling for graceful shutdown

- Add asio dependency for async signal handling
- Register SIGINT and SIGTERM to quit the application
- Poll signals each frame for non-blocking shutdown
- Q key and Ctrl+C both cleanly exit the program

CMakeLists.txt

@@ -11,9 +11,10 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/deps")
 find_package(fmt REQUIRED)
 find_package(glfw3 REQUIRED)
 find_package(glad REQUIRED)
+find_package(asio REQUIRED)
 
 # Target setup
 add_executable(cuber "cuber.cpp" "cbt/window.cpp")
 target_include_directories(cuber PRIVATE ".")
 target_compile_features(cuber PRIVATE cxx_std_23)
-target_link_libraries(cuber PRIVATE fmt::fmt glfw::glfw glad::glad)
+target_link_libraries(cuber PRIVATE fmt::fmt glfw::glfw glad::glad asio::asio)