make sure playbar center is centered

resources/playbar.blp

@@ -21,7 +21,7 @@ template $AudreyUiPlaybar: Adw.Bin {
       ]
 
       [start]
-      Box {
+      Box start {
         AspectFrame {
           visible: false; // FIXME annoying annoying annoying annoying
           // visible: bind template.show_cover_art;
@@ -156,7 +156,7 @@ template $AudreyUiPlaybar: Adw.Bin {
       }
 
       [end]
-      Box {
+      Box end {
         Button {
           icon-name: "heart-empty-symbolic"; // placeholder
           valign: center;
@@ -184,3 +184,9 @@ template $AudreyUiPlaybar: Adw.Bin {
     };
   };
 }
+
+// make sure the middle is centered always
+SizeGroup {
+  mode: horizontal;
+  widgets [start, end]
+}