extract layout.ex and cart.ex inline styles to CSS classes (Phase 3)

Move ~104 inline style= attributes from layout.ex (55) and cart.ex (49)
into named CSS classes in components.css. Remove conflicting unlayered
nav link rule from theme-semantic.css that was previously masked by
inline styles. Only dynamic values (background-image URLs, logo height)
remain as inline styles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/theme-semantic.css

@@ -312,8 +312,7 @@
   color: var(--t-text-primary);
 }
 
-.shop-nav a[aria-current="page"],
-.shop-nav span[aria-current="page"] {
-  color: var(--t-text-primary);
+/* Active nav underline — must stay unlayered to match the base border above */
+.shop-nav [aria-current="page"] {
   border-bottom-color: hsl(var(--t-accent-h) var(--t-accent-s) var(--t-accent-l));
 }