Add everything.

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.16)
+project(Modularity LANGUAGES C CXX)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+# ==================== WINDOWS FIXES (only active on Windows) ====================
+if(WIN32)
+    add_compile_definitions(
+        NOMINMAX                  # Fixes std::min/std::max clash with Windows.h
+        WIN32_LEAN_AND_MEAN       # Speeds up Windows.h includes
+        _CRT_SECURE_NO_WARNINGS   # Silences strncpy, sscanf, etc. warnings
+    )
+endif()
+
+# ==================== Compiler flags ====================
+if(MSVC)
+    add_compile_options(/W4 /O2 /permissive- /MP)
+else()
+    add_compile_options(-Wall -Wextra -Wpedantic -O2)
+endif()
+
+# ==================== Third-party libraries ====================
+
+add_subdirectory(src/ThirdParty/glfw EXCLUDE_FROM_ALL)
+find_package(OpenGL REQUIRED)
+
+# GLAD
+add_library(glad STATIC src/ThirdParty/glad/glad.c)
+target_include_directories(glad PUBLIC src/ThirdParty/glad)
+
+# GLM (header-only)
+add_library(glm INTERFACE)
+target_include_directories(glm INTERFACE src/ThirdParty/glm)
+
+# ImGuizmo ←←← THIS WAS THE BUG
+add_library(imguizmo STATIC
+    src/ThirdParty/ImGuizmo/ImGuizmo.cpp
+)
+target_include_directories(imguizmo PUBLIC src/ThirdParty/ImGuizmo)
+target_link_libraries(imguizmo PUBLIC imgui glm)   # ImGuizmo uses ImGui + GLM
+
+# Dear ImGui
+set(IMGUI_DIR ${PROJECT_SOURCE_DIR}/src/ThirdParty/imgui)
+add_library(imgui STATIC
+    ${IMGUI_DIR}/imgui.cpp
+    ${IMGUI_DIR}/imgui_demo.cpp
+    ${IMGUI_DIR}/imgui_draw.cpp
+    ${IMGUI_DIR}/imgui_tables.cpp
+    ${IMGUI_DIR}/imgui_widgets.cpp
+    ${IMGUI_DIR}/backends/imgui_impl_glfw.cpp
+    ${IMGUI_DIR}/backends/imgui_impl_opengl3.cpp
+)
+target_include_directories(imgui PUBLIC ${IMGUI_DIR} ${IMGUI_DIR}/backends)
+target_link_libraries(imgui PRIVATE glfw)
+
+# ==================== Your code ====================
+file(GLOB_RECURSE PROJECT_SOURCES
+    src/*.cpp
+    src/*.c
+)
+list(FILTER PROJECT_SOURCES EXCLUDE REGEX "src/ThirdParty/|src/main\\.cpp")
+
+add_library(core STATIC ${PROJECT_SOURCES})
+target_include_directories(core PUBLIC include)
+target_link_libraries(core PUBLIC glad glm imgui imguizmo)
+
+# ==================== Executable ====================
+add_executable(main src/main.cpp)
+target_link_libraries(main PRIVATE core glfw OpenGL::GL)
+
+# Optional: remove console window on Windows (uncomment if you want GUI-only app)
+#if(WIN32)
+#    set_target_properties(main PROPERTIES WIN32_EXECUTABLE TRUE)
+#endif()
+
+# Linux needs these extra libs
+if(NOT WIN32)
+    target_link_libraries(main PRIVATE dl pthread X11)
+endif()
+
+# ==================== Copy Resources folder after build ====================
+add_custom_command(TARGET main POST_BUILD
+    COMMAND ${CMAKE_COMMAND} -E copy_directory
+        ${CMAKE_SOURCE_DIR}/Resources
+        $<TARGET_FILE_DIR:main>/Resources
+)