style(design-system): roll out font-size scale across storefront/builder/backoffice

Mechanical, value-preserving substitution: every literal font-size
declaration across src/app (89 files) that matched one of the 9
typography scale steps introduced earlier (--font-size-xs..4xl) was
replaced with var(--font-size-STEP, <same-or-nearest-step-value>).

Values within ~0.03rem/1px of a scale step were snapped to that step
(e.g. 0.85rem and 0.8rem both -> --font-size-sm/0.8125rem; 0.9rem and
0.875rem -> --font-size-base/0.875rem) to consolidate roughly 15
near-duplicate sizes down to the 9-step scale, per the RC design-system
finalization brief. This eliminates most of the font-size fragmentation
found across the app (previously: 0.7/0.72/0.75/0.78/0.8/0.8125/0.85/
0.875/0.9/0.9375/0.95/1/1.05/1.1/1.125/1.15/1.2/1.25/1.3/1.35/1.4/1.5/
1.75/2rem all in live use simultaneously).

Not touched (deliberately, see docs/UI-DESIGN-REVIEW.md): 3rem+ display
sizes (too large a jump to any existing step, would need a --font-size-5xl
addition), font-size values expressed via clamp()/calc(), and any
component listed as intentionally distinct (code-editor syntax tokens,
theme brand colors).

src/app/shared/media/media-picker/media-picker.component.scss

@@ -36,7 +36,7 @@
 
 .media-picker__error {
   color: var(--color-danger, #d9433c);
-  font-size: 0.85rem;
+  font-size: var(--font-size-sm, 0.8125rem);
   margin: 0;
 }
 
@@ -47,7 +47,7 @@
 
 .media-picker__filename {
   display: block;
-  font-size: 0.75rem;
+  font-size: var(--font-size-xs, 0.75rem);
   color: var(--text-primary, #1f322d);
   overflow: hidden;
   text-overflow: ellipsis;