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>

tests/CMakeLists.txt

@@ -1,4 +1,18 @@
-find_package(GTest REQUIRED)
+# Try system GTest first, fall back to FetchContent
+find_package(GTest QUIET)
+
+if(NOT GTest_FOUND)
+    include(FetchContent)
+    FetchContent_Declare(
+        googletest
+        GIT_REPOSITORY https://github.com/google/googletest.git
+        GIT_TAG        v1.14.0
+        GIT_SHALLOW    TRUE
+    )
+    set(BUILD_GMOCK OFF CACHE BOOL "" FORCE)
+    set(INSTALL_GTEST OFF CACHE BOOL "" FORCE)
+    FetchContent_MakeAvailable(googletest)
+endif()
 
 # ──────────────────────────────────────────────
 # Pipeline unit tests