feat(fe): standardize empty data state with EmptyStatePlaceholder component and AGENTS.md rule

frontend/src/shared/ui/QueryStateBoundary.vue

@@ -2,6 +2,7 @@
 import KsButton from './components/KsButton.vue'
 import KsInlineMessage from './components/KsInlineMessage.vue'
 import SkeletonLoader from './components/SkeletonLoader.vue'
+import EmptyStatePlaceholder from './components/EmptyStatePlaceholder.vue'
 const props = defineProps<{
   loading: boolean
   processing?: boolean
@@ -34,7 +35,7 @@ const emit = defineEmits<{ retry: [] }>()
       <KsButton label="같은 요청 다시 시도" severity="secondary" @click="emit('retry')" />
       <small v-if="correlationId">Correlation: {{ correlationId }}</small>
     </div>
-    <KsInlineMessage v-else-if="props.empty" severity="info" message="표시할 데이터가 없습니다." />
+    <EmptyStatePlaceholder v-else-if="props.empty" />
     <template v-else>
       <KsInlineMessage v-if="props.partial" severity="warning" message="일부 데이터만 표시하고 있습니다. 완전성 경고를 확인하세요." />
       <KsInlineMessage v-if="props.warning" severity="warning" :message="props.warning" />