feat: add cart page, cart drawer, and shared cart infrastructure

- Cart context with pure functions for add/remove/update/hydrate
- Price formatting via ex_money (replaces all float division)
- CartHook on_mount with attach_hook for shared event handlers
  (open/close drawer, remove item, PubSub sync)
- Accessible cart drawer with focus trap, scroll lock, aria-live
- Cart page with increment/decrement quantity controls
- Preview mode cart drawer support in theme editor
- Cart persistence to session via JS hook + API endpoint
- 19 tests covering all Cart pure functions

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

assets/css/theme-semantic.css

@@ -220,10 +220,13 @@
   font-weight: 600;
   text-decoration: none;
   transition: top 0.2s ease;
+  outline: none;
+}
 
-  &:focus {
-    top: 1rem;
-  }
+.skip-link:focus {
+  top: 1rem;
+  outline: 3px solid var(--t-text-primary);
+  outline-offset: 2px;
 }
 
 /* Nav link styling with active state indicator */