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/project-editor/sections/widgets-section.component.scss

@@ -19,7 +19,7 @@
 .widget-card__title {
   margin: 0;
   font-size: var(--font-size-xl, 1.125rem);
-  font-weight: 600;
+  font-weight: var(--font-weight-semibold, 600);
   color: var(--text-primary);
   flex: 1 1 auto;
   min-width: 120px;
@@ -27,7 +27,7 @@
 
 .widget-card__hidden-badge {
   font-size: var(--font-size-xs, 0.75rem);
-  font-weight: 600;
+  font-weight: var(--font-weight-semibold, 600);
   letter-spacing: 0.4px;
   text-transform: uppercase;
   padding: 2px 8px;
@@ -90,7 +90,7 @@
   summary {
     cursor: pointer;
     font-size: var(--font-size-xs, 0.75rem);
-    font-weight: 600;
+    font-weight: var(--font-weight-semibold, 600);
     color: var(--text-light);
   }
 }