feat: About dialog with version, commit, build time and settings path

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

src/build_info.hpp

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: MIT
+#pragma once
+
+namespace udpscope::build_info {
+
+/// The CMake project version, e.g. "0.1.0".
+extern const char* const kVersion;
+/// `git describe --always --dirty` at build time, or "unknown" if git was not available.
+extern const char* const kCommit;
+/// When the binary was configured/built, as "YYYY-MM-DD HH:MM UTC".
+extern const char* const kTimestamp;
+
+}  // namespace udpscope::build_info