style(design-system): normalize font-weight literals (400/500/600/700) to tokens

Mechanical, value-preserving: font-weight: 400/500/600/700 -> var(--font-weight-normal/medium/semibold/bold, <same value>) across src/app.
Note: cart.component.scss now sits ~771 bytes over its per-file budget
in angular.json due to longer var() strings; non-fatal build warning,
noted in docs/UI-DESIGN-REVIEW.md as a follow-up (either bump the
component style budget slightly or accept the warning).

src/app/features/search/components/empty-results/search-empty-results.component.scss

@@ -17,7 +17,7 @@ h3 {
   color: var(--text-primary);
   padding: 0 14px;
   cursor: pointer;
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
 }
 
 .block {

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

@@ -51,7 +51,7 @@
   border-radius: var(--radius-sm);
   background: var(--primary-color);
   color: #fff;
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
   padding: 0 14px;
   cursor: pointer;
 }
@@ -108,7 +108,7 @@
 }
 
 .suggestion-title {
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
 }
 
 .suggestion-copy small,
@@ -145,7 +145,7 @@
   background: transparent;
   color: var(--primary-color);
   cursor: pointer;
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
 }
 
 .no-results {