feat: keyboard and mouse shortcut overview window

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

src/help_window.hpp

@@ -28,6 +28,11 @@ struct HelpReport {
 /// Loads and validates every page without building a window. Safe to call headless.
 HelpReport help_self_check();
 
+/// The shortcut overview: the application accelerators and the plot's own interactions,
+/// in the sections the tables name. Hand-rolled because GtkShortcutsWindow is deprecated
+/// as of GTK 4.18 and this project builds warning-free.
+void show_shortcuts(GtkWindow* parent);
+
 /// Closes whichever of the three windows are open, via gtk_window_close() so the
 /// close-request handlers run and the pointers are nulled. Used by the probe.
 void close_help_windows();