fix(app): use four pixel indicator margin

packages/app/src/components/titlebar-tab-nav.css

@@ -13,9 +13,9 @@
   &::after {
     content: "";
     position: absolute;
-    right: 8px;
+    right: 4px;
     bottom: 0;
-    left: 8px;
+    left: 4px;
     height: 1px;
     pointer-events: none;
     border-radius: 2px 2px 0 0;