Migrate build system from qmake/makefiles to CMake

Remove legacy build files (Makefile, makefile.unix/mingw/osx,
triangles-qt.pro) and replace with CMake build system. Includes
find modules for all dependencies, LevelDB bundled build, and
updated CI workflow for CMake + Ninja on all platforms.

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

cmake/GenerateBuildInfo.cmake

@@ -0,0 +1,19 @@
+# cmake/GenerateBuildInfo.cmake
+# Sets up a custom target that generates build.h from git describe,
+# equivalent to share/genbuild.sh.
+
+set(BUILD_HEADER_DIR "${CMAKE_BINARY_DIR}/generated")
+set(BUILD_HEADER     "${BUILD_HEADER_DIR}/build.h")
+file(MAKE_DIRECTORY  "${BUILD_HEADER_DIR}")
+
+# Custom command runs on every build to regenerate build.h if git state changed
+add_custom_target(generate_build_info ALL
+    COMMAND ${CMAKE_COMMAND}
+        -DSOURCE_DIR=${CMAKE_SOURCE_DIR}
+        -DOUTPUT_FILE=${BUILD_HEADER}
+        -P "${CMAKE_SOURCE_DIR}/cmake/GenerateBuildInfoScript.cmake"
+    WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
+    COMMENT "Generating build.h from git describe..."
+    BYPRODUCTS "${BUILD_HEADER}"
+    VERBATIM
+)