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/lib/components/DocumentCard.svelte

@@ -2,7 +2,7 @@
   import FormatIcon from './FormatIcon.svelte';
   import TagPill from './TagPill.svelte';
 
-  let { doc, showDomain = true } = $props();
+  let { doc, showDomain = true, selected = false, onselect = null } = $props();
 
   function formatDate(dateStr) {
     if (!dateStr) return '';
@@ -23,9 +23,13 @@
   }
 </script>
 
-<a
-  href="/documents/{doc.id}"
-  class="flex items-start gap-3 p-3 bg-[var(--surface)] border border-[var(--border)] rounded-lg hover:border-[var(--accent)] transition-colors group"
+{@const Tag = onselect ? 'button' : 'a'}
+<svelte:element
+  this={Tag}
+  href={onselect ? undefined : `/documents/${doc.id}`}
+  onclick={onselect ? (e) => { e.preventDefault(); onselect(doc); } : undefined}
+  class="flex items-start gap-3 p-3 bg-[var(--surface)] border rounded-lg hover:border-[var(--accent)] transition-colors group w-full text-left
+    {selected ? 'border-[var(--accent)] bg-[var(--accent)]/5' : 'border-[var(--border)]'}"
 >
   <!-- 포맷 아이콘 -->
   <div class="shrink-0 mt-0.5 text-[var(--text-dim)] group-hover:text-[var(--accent)]">
@@ -76,4 +80,4 @@
       <span class="text-[var(--text-dim)]">{formatSize(doc.file_size)}</span>
     {/if}
   </div>
-</a>
+</svelte:element>

frontend/src/lib/components/PreviewPanel.svelte

@@ -0,0 +1,119 @@
+<script>
+  import { X, ExternalLink } from 'lucide-svelte';
+  import FormatIcon from './FormatIcon.svelte';
+  import TagPill from './TagPill.svelte';
+
+  let { doc, onclose } = $props();
+
+  function formatDate(dateStr) {
+    if (!dateStr) return '-';
+    return new Date(dateStr).toLocaleDateString('ko-KR', { year: 'numeric', month: 'short', day: 'numeric' });
+  }
+
+  function formatSize(bytes) {
+    if (!bytes) return '-';
+    if (bytes < 1024) return `${bytes}B`;
+    if (bytes < 1048576) return `${(bytes / 1024).toFixed(0)}KB`;
+    return `${(bytes / 1048576).toFixed(1)}MB`;
+  }
+</script>
+
+<aside class="h-full flex flex-col bg-[var(--sidebar-bg)] border-l border-[var(--border)] overflow-y-auto">
+  <!-- 헤더 -->
+  <div class="flex items-center justify-between px-4 py-3 border-b border-[var(--border)] shrink-0">
+    <div class="flex items-center gap-2 min-w-0">
+      <FormatIcon format={doc.file_format} size={16} />
+      <span class="text-sm font-medium truncate">{doc.title || '제목 없음'}</span>
+    </div>
+    <button onclick={onclose} class="p-1 rounded hover:bg-[var(--surface)] text-[var(--text-dim)]" aria-label="닫기">
+      <X size={16} />
+    </button>
+  </div>
+
+  <div class="flex-1 p-4 space-y-4">
+    <!-- 전체 보기 버튼 -->
+    <a
+      href="/documents/{doc.id}"
+      class="flex items-center justify-center gap-2 w-full px-3 py-2 bg-[var(--accent)] text-white text-sm rounded-lg hover:bg-[var(--accent-hover)] transition-colors"
+    >
+      <ExternalLink size={14} />
+      전체 보기
+    </a>
+
+    <!-- AI 요약 -->
+    {#if doc.ai_summary}
+      <div>
+        <h4 class="text-xs font-semibold text-[var(--text-dim)] uppercase mb-1.5">AI 요약</h4>
+        <p class="text-sm leading-relaxed">{doc.ai_summary}</p>
+      </div>
+    {/if}
+
+    <!-- 태그 -->
+    {#if doc.ai_tags?.length}
+      <div>
+        <h4 class="text-xs font-semibold text-[var(--text-dim)] uppercase mb-1.5">태그</h4>
+        <div class="flex flex-wrap gap-1">
+          {#each doc.ai_tags as tag}
+            <TagPill {tag} />
+          {/each}
+        </div>
+      </div>
+    {/if}
+
+    <!-- 메타 정보 -->
+    <div>
+      <h4 class="text-xs font-semibold text-[var(--text-dim)] uppercase mb-1.5">정보</h4>
+      <dl class="space-y-1.5 text-xs">
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">포맷</dt>
+          <dd class="uppercase">{doc.file_format}</dd>
+        </div>
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">크기</dt>
+          <dd>{formatSize(doc.file_size)}</dd>
+        </div>
+        {#if doc.ai_domain}
+          <div class="flex justify-between">
+            <dt class="text-[var(--text-dim)]">분류</dt>
+            <dd class="text-right">{doc.ai_domain.replace('Knowledge/', '')}{doc.ai_sub_group ? ` / ${doc.ai_sub_group}` : ''}</dd>
+          </div>
+        {/if}
+        {#if doc.source_channel}
+          <div class="flex justify-between">
+            <dt class="text-[var(--text-dim)]">출처</dt>
+            <dd>{doc.source_channel}</dd>
+          </div>
+        {/if}
+        {#if doc.data_origin}
+          <div class="flex justify-between">
+            <dt class="text-[var(--text-dim)]">구분</dt>
+            <dd>{doc.data_origin}</dd>
+          </div>
+        {/if}
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">등록일</dt>
+          <dd>{formatDate(doc.created_at)}</dd>
+        </div>
+      </dl>
+    </div>
+
+    <!-- 처리 상태 -->
+    <div>
+      <h4 class="text-xs font-semibold text-[var(--text-dim)] uppercase mb-1.5">처리</h4>
+      <dl class="space-y-1 text-xs">
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">추출</dt>
+          <dd class={doc.extracted_at ? 'text-[var(--success)]' : 'text-[var(--text-dim)]'}>{doc.extracted_at ? '완료' : '대기'}</dd>
+        </div>
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">분류</dt>
+          <dd class={doc.ai_processed_at ? 'text-[var(--success)]' : 'text-[var(--text-dim)]'}>{doc.ai_processed_at ? '완료' : '대기'}</dd>
+        </div>
+        <div class="flex justify-between">
+          <dt class="text-[var(--text-dim)]">임베딩</dt>
+          <dd class={doc.embedded_at ? 'text-[var(--success)]' : 'text-[var(--text-dim)]'}>{doc.embedded_at ? '완료' : '대기'}</dd>
+        </div>
+      </dl>
+    </div>
+  </div>
+</aside>

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>