fix(web): make ShopOfferView.effectSummary/requirements required

The server always sends both fields; an optional frontend type only
dodged a transient gap in merchant.store.spec.ts's shopView() fixture
and risked a requirement line silently failing to render if a future
fixture omitted the fields. Add the two fields to that single fixture
instead (an open offer, so null/[] are the honest values) and drop the
now-unnecessary `?? []` tolerance in the template.

apps/web/src/app/core/api/game-api.models.ts

@@ -446,13 +446,8 @@ export interface ShopOfferView {
   currencyType: string;
   price: number;
   quantity: number;
-  /**
-   * Optional so fixtures written before slice 0.8.5 (e.g. the trade-in store's
-   * own spec, out of this task's scope) keep compiling untouched. The server
-   * always sends both fields; the template treats a missing one as "none".
-   */
-  effectSummary?: string | null;
-  requirements?: ShopOfferRequirement[];
+  effectSummary: string | null;
+  requirements: ShopOfferRequirement[];
   unlocked: boolean;
   affordable: boolean;
 }

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

@@ -228,7 +228,7 @@
                   }}</span>
                 }
                 @for (
-                  requirement of offer.requirements ?? [];
+                  requirement of offer.requirements;
                   track requirement.label
                 ) {
                   <span

apps/web/src/app/features/npc/merchant.store.spec.ts

@@ -89,6 +89,8 @@ function shopView(): ShopView {
         currencyType: 'SILVER',
         price: 12,
         quantity: 1,
+        effectSummary: null,
+        requirements: [],
         unlocked: true,
         affordable: true,
       },