fix: --clamp argparse missing .append() blocks multi-screen apply (#577)

Multi-screen mode requires one --clamp per --screen-root, but the
argparse registration only allowed a single occurrence — passing two
crashes with "Duplicate argument --clamp". Mirror what --scaling
already does and append the modifier.

Co-authored-by: kuro-blk <kuro@example.com>

src/WallpaperEngine/Application/ApplicationContext.cpp

@@ -394,7 +394,8 @@ void ApplicationContext::loadSettingsFromArgv () {
 	    } else {
 		this->settings.render.window.clamp = flags;
 	    }
-	});
+	})
+	.append ();
 
     backgroundGroup.add_argument ("--layer")
 	.help (