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/header/header.component.scss

@@ -174,7 +174,7 @@
   top: -6px;
   min-width: 16px;
   height: 16px;
-  border-radius: 999px;
+  border-radius: var(--radius-full, 999px);
   background: #497671;
   color: #ffffff;
   font-size: 10px;
@@ -273,7 +273,7 @@
   padding: 15px 26px;
   background: #e9edf1;
   border: 1px solid #d3dad9;
-  border-radius: 13px;
+  border-radius: var(--radius-lg, 13px);
   box-shadow: 0 3px 4px 0 rgba(0, 0, 0, 0.15);
   font-family: 'DM Sans', sans-serif;
   font-size: var(--font-size-lg, 1rem);