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

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

src/help_window.cpp

@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: MIT
+#include "help_window.hpp"
+
+#include <cstdio>
+
+#include "build_info.hpp"
+#include "layout_store.hpp"
+
+namespace udpscope {
+namespace {
+
+/// Nulled by the `close-request` handler so a second open cannot present a dead widget.
+/// One pointer per window, file-scope because there is one application.
+GtkWidget* g_about = nullptr;
+
+gboolean on_window_close(GtkWindow* window, gpointer slot) {
+  (void)window;
+  *static_cast<GtkWidget**>(slot) = nullptr;
+  return FALSE;  // let the default handler destroy it
+}
+
+}  // namespace
+
+void show_about(GtkWindow* parent, const std::string& session_path) {
+  if (g_about != nullptr) {
+    gtk_window_present(GTK_WINDOW(g_about));
+    return;
+  }
+
+  // GtkAboutDialog is not deprecated in GTK 4 (unlike GtkShortcutsWindow), so the
+  // stock dialog is used here and the overview is hand-rolled.
+  GtkWidget* dialog = gtk_about_dialog_new();
+  g_about = dialog;
+  GtkAboutDialog* about = GTK_ABOUT_DIALOG(dialog);
+  gtk_about_dialog_set_program_name(about, "udpscope");
+  gtk_about_dialog_set_version(about, build_info::kVersion);
+  gtk_about_dialog_set_comments(about, "An oscilloscope for UDPStreamer sources.");
+  gtk_about_dialog_set_license_type(about, GTK_LICENSE_MIT_X11);
+
+  char details[1024];
+  std::snprintf(details, sizeof(details),
+                "Commit: %s\nBuilt: %s\nSettings: %s\nSession file: %s",
+                build_info::kCommit, build_info::kTimestamp, config_dir().c_str(),
+                session_path.c_str());
+  gtk_about_dialog_set_system_information(about, details);
+
+  gtk_window_set_transient_for(GTK_WINDOW(dialog), parent);
+  gtk_window_set_modal(GTK_WINDOW(dialog), TRUE);
+  g_signal_connect(dialog, "close-request", G_CALLBACK(on_window_close), &g_about);
+  gtk_window_present(GTK_WINDOW(dialog));
+}
+
+}  // namespace udpscope