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/shared/ui/code-editor/code-editor.component.scss

@@ -66,7 +66,7 @@
 
 :global(.cm-selector) {
   color: #b2452f;
-  font-weight: 600;
+  font-weight: var(--font-weight-semibold, 600);
 }
 
 :global(.cm-property) {
@@ -79,7 +79,7 @@
 
 :global(.cm-keyword) {
   color: #6d3fa8;
-  font-weight: 600;
+  font-weight: var(--font-weight-semibold, 600);
 }
 
 :global(.cm-punct) {