style(design-system): normalize exact-match border-radius literals to tokens

Mechanical sweep for border-radius: 4px/8px/12px/13px/999px replaced
with var(--radius-xs/sm/md/lg/full, <same value>) across src/app.
Only exact matches to existing token values were touched (20px, 16px,
10px, 6px, 3px, 2px etc. were left as-is since no token maps to them
without a visible size change on at least one tenant theme — see
docs/UI-DESIGN-REVIEW.md).

src/app/components/footer/footer.component.scss

@@ -69,7 +69,7 @@
   flex-wrap: wrap;
 
   img {
-    border-radius: 4px;
+    border-radius: var(--radius-xs, 4px);
     background: #fff;
     padding: 2px 6px;
   }