feat(web): announce offers a trade just unlocked

apps/web/src/app/features/npc/merchant-page.component.html

@@ -208,6 +208,21 @@
         <h2 class="merchant__panel-title">{{ shop.shopName }}</h2>
         <p class="merchant__purse" data-purse>{{ shop.silver }} Silver</p>
 
+        @if (store.newlyUnlocked().length > 0) {
+          <p class="shop-unlocked" data-unlocked aria-live="polite">
+            @for (name of store.newlyUnlocked(); track name) {
+              <span>New merchant offer unlocked: {{ name }}</span>
+            }
+            <button
+              type="button"
+              class="merchant__link"
+              (click)="store.dismissUnlocked()"
+            >
+              Dismiss
+            </button>
+          </p>
+        }
+
         <ul class="shop-list">
           @for (offer of shop.offers; track offer.itemKey) {
             <li class="shop-row" [attr.data-shop-item]="offer.itemKey">