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/backoffice/media/components/asset-tile/asset-tile.component.scss

@@ -55,7 +55,7 @@
 
 .asset-tile__filename {
   font-size: var(--font-size-sm, 0.8125rem);
-  font-weight: 700;
+  font-weight: var(--font-weight-bold, 700);
   color: var(--text-primary, #1e3c38);
   overflow: hidden;
   text-overflow: ellipsis;

src/app/features/backoffice/shared/backoffice-coming-soon-page.component.scss

@@ -23,7 +23,7 @@
 
 .coming-soon__link {
   color: var(--primary-color, #497671);
-  font-weight: 600;
+  font-weight: var(--font-weight-semibold, 600);
   text-decoration: none;
 }