Restore GTip's border radius to the theme value

Porting GTip off SCSS variables replaced $border-radius-base with a
hardcoded 0.25rem, but $border-radius-base is 0.1875rem, so every variant
gained a pixel of corner radius. The code block lower in the same file
still rounds to 3px, which is what 0.1875rem resolves to.

The color changes in that port are a different matter and are left as
they are: GTip's success/warning/danger swatches now come from the token
palette rather than the $brand-* variables, which is what makes GTip
consistent with GButton and is presumably the point of the move.

client/packages/ui/src/components/GTip.vue

@@ -2,7 +2,6 @@
 import { computed, onUnmounted, ref } from "vue";
 
 import { useMarkdown } from "../composables/markdown";
-
 import type { ColorVariant } from "./componentVariants";
 
 interface Props {
@@ -90,7 +89,7 @@ onUnmounted(() => {
     position: relative;
     padding: 0.5rem 0.75rem;
     background-color: var(--color-grey-100);
-    border-radius: 0.25rem;
+    border-radius: 0.1875rem;
     border-left: 3px solid var(--color-blue-600);
 
     &.tip-info {