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/search/components/empty-results/search-empty-results.component.scss

@@ -34,7 +34,7 @@ h3 {
 .chip {
   min-height: 36px;
   border: 1px solid var(--border-color);
-  border-radius: 999px;
+  border-radius: var(--radius-full, 999px);
   background: var(--bg-primary);
   color: var(--text-secondary);
   padding: 0 12px;

src/app/features/search/components/search-bar/search-bar.component.scss

@@ -126,7 +126,7 @@
 .chip {
   min-height: 32px;
   border: 1px solid var(--border-color);
-  border-radius: 999px;
+  border-radius: var(--radius-full, 999px);
   background: var(--bg-primary);
   color: var(--text-secondary);
   padding: 0 10px;

src/app/features/search/components/trending-searches/trending-searches.component.scss

@@ -12,7 +12,7 @@
 .chip {
   min-height: 32px;
   border: 1px solid var(--border-color);
-  border-radius: 999px;
+  border-radius: var(--radius-full, 999px);
   background: var(--bg-primary);
   color: var(--text-secondary);
   padding: 0 10px;