fix: make project compilable on Ubuntu with Qt 6.4 and no GTest

- Lower Qt6 minimum version requirement from 6.8 to 6.4
- tests/CMakeLists.txt: fall back to FetchContent for GTest if not found
- Add MIT LICENSE file (required by CPack)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CMakeLists.txt

@@ -56,7 +56,7 @@ else()
 endif()
 
 if(BUILD_GUI)
-    find_package(Qt6 6.8 REQUIRED COMPONENTS Widgets)
+    find_package(Qt6 6.4 REQUIRED COMPONENTS Widgets)
     qt_standard_project_setup()
 endif()