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/api/src/shops/shop.service.ts

@@ -307,9 +307,11 @@ export class ShopService {
         definitionId: offer.lootBagDefinition.id,
         key: offer.lootBagDefinition.key,
         name: offer.lootBagDefinition.name,
-        // A bag definition carries no flavour text of its own; the capacity
-        // line is the honest description of what it is.
-        description: describeBagEffect(offer.lootBagDefinition),
+        // A bag definition carries no flavour text of its own. The capacity
+        // line is the whole of what there is to say about it, and it is already
+        // carried by `effectSummary`; repeating it here would render it twice
+        // (slice §5 shows the line once).
+        description: '',
         iconPath: offer.lootBagDefinition.iconPath,
         effectSummary: describeBagEffect(offer.lootBagDefinition),
       };