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/components/delivery-selector/delivery-selector.component.scss

@@ -14,7 +14,7 @@
   display: block;
   margin-bottom: 8px;
   font-size: var(--font-size-sm, 0.8125rem);
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
   color: #3f5f5c;
 }
 
@@ -50,7 +50,7 @@
   padding: 6px 10px;
   border-radius: var(--radius-full, 999px);
   font-size: var(--font-size-sm, 0.8125rem);
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
 }
 
 .delivery-chip--digital {