fix(shop): render a bag offer's capacity line once, not twice

`resolveTarget` gave a bag target the same string for `description` and
`effectSummary`, and the shop row renders both, so the slice's two flagship
offers showed "Capacity: 5 Raider Trophies" on consecutive lines. Spec §5's
worked example shows it once.

A bag definition carries no flavour text of its own, so the description is now
empty and the row omits the span entirely rather than emitting an empty one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -234,9 +234,11 @@
               />
               <div class="shop-row__naming">
                 <span class="shop-row__name">{{ offer.itemName }}</span>
-                <span class="shop-row__description">{{
-                  offer.itemDescription
-                }}</span>
+                @if (offer.itemDescription) {
+                  <span class="shop-row__description">{{
+                    offer.itemDescription
+                  }}</span>
+                }
                 @if (offer.effectSummary) {
                   <span class="shop-row__effect" data-effect>{{
                     offer.effectSummary