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/features/content-management/components/content-health-widget/content-health-widget.component.scss

@@ -24,7 +24,7 @@
 
 .health-widget__bar {
   height: 8px;
-  border-radius: 999px;
+  border-radius: var(--radius-full, 999px);
   background: var(--surface-muted, #eef2f0);
   overflow: hidden;
 }

src/app/features/content-management/components/page-card/page-card.component.scss

@@ -15,7 +15,7 @@
   &:focus-visible {
     outline: 2px solid var(--brand-primary, #1e8a6e);
     outline-offset: 3px;
-    border-radius: 8px;
+    border-radius: var(--radius-sm, 8px);
   }
 }
 

src/app/features/content-management/components/page-editor/page-editor.component.scss

@@ -101,7 +101,7 @@
   width: 100%;
   max-height: 140px;
   object-fit: cover;
-  border-radius: 8px;
+  border-radius: var(--radius-sm, 8px);
 }
 
 .media-block__actions {