fix(slint): trim header nav-tab width by 6px

Drop 2px of left padding and 4px of right padding (after the caret) on
the Discover / Library / Local Library / My QBZ tabs.

cc

crates/qbz-slint/ui/shell/HeaderBar.slint

@@ -58,8 +58,8 @@ component NavTab inherits Rectangle {
         : ta.has-hover ? Theme.surface-hover : transparent;
 
     tab-row := HorizontalLayout {
-        padding-left: root.show-icon ? 11px : 13px;
-        padding-right: 11px;
+        padding-left: root.show-icon ? 9px : 11px;
+        padding-right: 7px;
         spacing: 6px;
         if root.show-icon: VerticalLayout {
             alignment: center;