feat: Phase 1C — 프리뷰 패널 (문서 선택 시 우측 표시)

- PreviewPanel: AI 요약, 태그, 메타 정보, 처리 상태 표시
- DocumentCard: 선택 모드 지원 (클릭→프리뷰, 더블클릭 불필요)
- 3-pane 완성: sidebar | document list | preview panel
- 필터 변경 시 선택 자동 해제
- 데스크톱만 표시 (모바일은 detail 페이지로 이동)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/documents/+page.svelte

@@ -4,8 +4,10 @@
   import { api } from '$lib/api';
   import { addToast } from '$lib/stores/ui';
   import DocumentCard from '$lib/components/DocumentCard.svelte';
+  import PreviewPanel from '$lib/components/PreviewPanel.svelte';
 
   let documents = $state([]);
+  let selectedDoc = $state(null);
   let total = $state(0);
   let loading = $state(true);
   let searchQuery = $state('');
@@ -29,6 +31,8 @@
     searchQuery = urlQ;
     searchMode = urlMode;
 
+    selectedDoc = null; // 필터 변경 시 선택 해제
+
     if (urlQ) {
       doSearch(urlQ, urlMode);
     } else {
@@ -112,9 +116,15 @@
   let totalPages = $derived(Math.ceil(total / 20));
   let items = $derived(searchResults || documents);
   let hasActiveFilters = $derived(!!filterDomain || !!filterSubGroup || !!searchQuery);
+
+  function selectDoc(doc) {
+    selectedDoc = selectedDoc?.id === doc.id ? null : doc;
+  }
 </script>
 
-<div class="p-4 lg:p-6">
+<div class="flex h-full">
+<!-- 문서 목록 영역 -->
+<div class="flex-1 overflow-y-auto p-4 lg:p-6">
   <!-- 검색바 + 필터 상태 -->
   <div class="flex gap-2 mb-4">
     <input
@@ -183,7 +193,12 @@
   {:else}
     <div class="space-y-1.5">
       {#each items as doc}
-        <DocumentCard {doc} showDomain={!filterDomain} />
+        <DocumentCard
+          {doc}
+          showDomain={!filterDomain}
+          selected={selectedDoc?.id === doc.id}
+          onselect={selectDoc}
+        />
       {/each}
     </div>
 
@@ -203,3 +218,11 @@
     {/if}
   {/if}
 </div>
+
+<!-- 프리뷰 패널 (데스크톱만) -->
+{#if selectedDoc}
+  <div class="hidden lg:block shrink-0" style="width: 360px">
+    <PreviewPanel doc={selectedDoc} onclose={() => selectedDoc = null} />
+  </div>
+{/if}
+</div>