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

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

src/help_window.hpp

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: MIT
+#pragma once
+
+#include <gtk/gtk.h>
+
+#include <string>
+
+namespace udpscope {
+
+/// The About dialog: version, commit, build timestamp and where the session files live.
+/// Modal on `parent`. `session_path` is this run's own session file.
+void show_about(GtkWindow* parent, const std::string& session_path);
+
+}  // namespace udpscope