Fix notification circle (border-radius) (#26794)

`border-radius` means `radius`, not `diameter`, so it should be `50%` and `boxHeight / 2`

web_src/css/base.css

@@ -18,7 +18,7 @@
   /* non-color variables */
   --border-radius: 4px;
   --border-radius-medium: 6px;
-  --border-radius-circle: 100%;
+  --border-radius-circle: 50%;
   --opacity-disabled: 0.55;
   --height-loading: 16rem;
   --tab-size: 4;

web_src/css/modules/navbar.css

@@ -133,8 +133,8 @@
   left: 6px;
   top: -9px;
   min-width: 17px;
-  min-height: 17px;
-  border-radius: var(--border-radius-circle);
+  height: 17px;
+  border-radius: 11px; /* (height + 2 * borderThickness) / 2 */
   display: flex;
   align-items: center;
   justify-content: center;