License Effector under the GPL, version 3 or later

LICENSE holds the text, the README and About Effector state it, and the
Arch package declares it. The version moves to 0.5.2 for the release.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/gui/window.cpp

@@ -2221,6 +2221,8 @@ void EffectorWindow::showAbout() {
         aboutDialog_->set_version(std::string(kVersion));
         aboutDialog_->set_comments("Image effects, cascaded through a chain.");
         aboutDialog_->set_logo_icon_name("applications-graphics");
+        aboutDialog_->set_copyright("Copyright © 2026 Si");
+        aboutDialog_->set_license_type(Gtk::License::GPL_3_0);
     }
     aboutDialog_->present();
 }