fix(MyRewards): 调整奖励卡片商户名称显示样式

将商户名称从标题行分离，单独显示为灰色小号文本，提升可读性

@@ -33,7 +33,10 @@
         >
           <image :src="reward.thumbnail || 'https://placehold.co/120x120/e2f3ff/0369a1?text=券&font=roboto'" class="w-16 h-16 rounded-lg mr-4 flex-shrink-0" mode="aspectFill" />
           <view class="flex-1 min-w-0">
-            <view class="font-medium text-base mb-1">{{ reward.merchant_name }}·{{ reward.title }}</view>
+            <view class="font-medium text-base mb-1">{{ reward.title }}</view>
+            <view v-if="reward.merchant_name" class="text-xs text-gray-400 mb-1">
+              {{ reward.merchant_name }}
+            </view>
             <view v-if="reward.status === 'UNUSED'" class="text-xs text-gray-400">
               有效期至：{{ formatDate(reward.expire_time) }}
             </view>