feat(ui): Phase D.1 — 3-panel layout + DocumentMetaRail + useMedia

- 가로 flex 최상위 + 가운데 flex-1 (기존 list/viewer 세로 split 그대로 보존)
- xl+ (≥1280px): 우측 320px persistent rail, 접기 시 40px sliver.
  localStorage.metaRailOpen 으로 상태 유지.
- < xl : 기존 수동 drawer 제거하고 ui/Drawer primitive + uiState 사용.
- 리사이즈 시 xl+ 진입하면 drawer 자동 close (rail로 승계).
- handleKeydown → ui.handleEscape() 로 중앙화.
- ℹ 버튼 token 기반 재작성 (isXl 분기로 rail/drawer 토글).
- PreviewPanel.svelte 한 글자도 수정 없음 (Phase E 영역).

신규:
- frontend/src/lib/composables/useMedia.svelte.ts — matchMedia runes 컴포저블
- frontend/src/lib/components/DocumentMetaRail.svelte — PreviewPanel wrapper

검증:
- npm run build 통과
- npm run lint:tokens 241 → 236 (신규 코드 0 위반, 레거시 drawer/ℹ 버튼
  제거로 5건 organically 감소)
- PreviewPanel diff 0줄

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

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

@@ -3,13 +3,15 @@
   import { goto } from '$app/navigation';
   import { api } from '$lib/api';
   import { addToast } from '$lib/stores/toast';
-  import { Info } from 'lucide-svelte';
-  import { List, LayoutGrid } from 'lucide-svelte';
+  import { Info, List, LayoutGrid, ChevronLeft } from 'lucide-svelte';
   import DocumentCard from '$lib/components/DocumentCard.svelte';
   import DocumentTable from '$lib/components/DocumentTable.svelte';
-  import PreviewPanel from '$lib/components/PreviewPanel.svelte';
   import DocumentViewer from '$lib/components/DocumentViewer.svelte';
+  import DocumentMetaRail from '$lib/components/DocumentMetaRail.svelte';
   import UploadDropzone from '$lib/components/UploadDropzone.svelte';
+  import Drawer from '$lib/components/ui/Drawer.svelte';
+  import { ui } from '$lib/stores/uiState.svelte';
+  import { useIsXl } from '$lib/composables/useMedia.svelte';
 
   // 뷰 모드 (localStorage 기억)
   let viewMode = $state(typeof localStorage !== 'undefined' ? (localStorage.getItem('viewMode') || 'card') : 'card');
@@ -25,7 +27,47 @@
   let searchMode = $state('hybrid');
   let searchResults = $state(null);
   let selectedDoc = $state(null);
-  let infoPanelOpen = $state(false);
+
+  // D.1: 3-패널 레이아웃 상태.
+  // - xl+ : inline rail 사용 (metaRailOpen, localStorage persistent)
+  // - < xl: Drawer 프리미티브 사용 (ui.openDrawer('meta'))
+  const isXl = useIsXl();
+
+  let metaRailOpen = $state(
+    typeof localStorage !== 'undefined'
+      ? localStorage.getItem('metaRailOpen') !== 'false'
+      : true
+  );
+
+  function setMetaRailOpen(v) {
+    metaRailOpen = v;
+    if (typeof localStorage !== 'undefined') {
+      localStorage.setItem('metaRailOpen', String(v));
+    }
+  }
+
+  function toggleInfoPanel() {
+    if (!selectedDoc) return;
+    if (isXl.current) {
+      setMetaRailOpen(!metaRailOpen);
+    } else {
+      if (ui.isDrawerOpen('meta')) ui.closeDrawer();
+      else ui.openDrawer('meta');
+    }
+  }
+
+  function handleDocDelete() {
+    selectedDoc = null;
+    if (ui.isDrawerOpen('meta')) ui.closeDrawer();
+    loadDocuments();
+  }
+
+  // xl+ 진입 시 열려있던 drawer는 자동으로 닫는다 (rail로 승계).
+  $effect(() => {
+    if (isXl.current && ui.isDrawerOpen('meta')) {
+      ui.closeDrawer();
+    }
+  });
 
   // URL params → filter
   let currentPage = $derived(parseInt($page.url.searchParams.get('page') || '1'));
@@ -46,7 +88,7 @@
     searchQuery = urlQ;
     searchMode = urlMode;
     selectedDoc = null;
-    infoPanelOpen = false;
+    if (ui.isDrawerOpen('meta')) ui.closeDrawer();
 
     if (urlQ) {
       doSearch(urlQ, urlMode);
@@ -139,8 +181,8 @@
   }
 
   function handleKeydown(e) {
-    if (e.key === 'Escape' && infoPanelOpen) {
-      infoPanelOpen = false;
+    if (e.key === 'Escape') {
+      ui.handleEscape(); // drawer/modal stack 우선순위로 중앙 처리
     }
   }
 
@@ -151,7 +193,9 @@
 
 <svelte:window on:keydown={handleKeydown} />
 
-<div class="flex flex-col h-full">
+<div class="flex h-full">
+  <!-- 가운데: 기존 list + viewer 세로 split (변경 없음) -->
+  <div class="flex-1 flex flex-col min-h-0">
   <!-- 상단 영역 -->
   <div class="{selectedDoc ? 'h-[30%] shrink-0 border-b border-[var(--border)]' : 'flex-1'} flex flex-col min-h-0">
     <!-- 업로드 드롭존 -->
@@ -190,11 +234,15 @@
         {/if}
       </button>
       {#if selectedDoc}
+        {@const isPanelActive = isXl.current ? metaRailOpen : ui.isDrawerOpen('meta')}
         <button
-          onclick={() => infoPanelOpen = !infoPanelOpen}
-          class="p-1.5 rounded-lg border border-[var(--border)] hover:border-[var(--accent)] text-[var(--text-dim)] hover:text-[var(--accent)] transition-colors
-            {infoPanelOpen ? 'bg-[var(--accent)]/10 border-[var(--accent)] text-[var(--accent)]' : ''}"
+          type="button"
+          onclick={toggleInfoPanel}
+          class="p-1.5 rounded-lg border transition-colors {isPanelActive
+            ? 'border-accent text-accent bg-accent/10'
+            : 'border-default text-dim hover:text-accent hover:border-accent'}"
           aria-label="문서 정보"
+          aria-pressed={isPanelActive}
           title="문서 정보"
         >
           <Info size={16} />
@@ -288,18 +336,43 @@
       <DocumentViewer doc={selectedDoc} />
     </div>
   {/if}
+  </div>
+  <!-- /가운데 컬럼 -->
+
+  <!-- 우측: xl+ persistent rail (hidden xl:flex) -->
+  {#if selectedDoc}
+    <aside
+      class="hidden xl:flex shrink-0 border-l border-default {metaRailOpen ? 'w-rail' : 'w-10'}"
+      aria-label="문서 정보 rail"
+    >
+      {#if metaRailOpen}
+        <DocumentMetaRail
+          doc={selectedDoc}
+          onclose={() => setMetaRailOpen(false)}
+          ondelete={handleDocDelete}
+        />
+      {:else}
+        <button
+          type="button"
+          onclick={() => setMetaRailOpen(true)}
+          class="w-10 h-full flex items-center justify-center text-dim hover:text-text hover:bg-surface-hover transition-colors"
+          aria-label="정보 패널 펼치기"
+          title="정보 패널 펼치기"
+        >
+          <ChevronLeft size={16} />
+        </button>
+      {/if}
+    </aside>
+  {/if}
 </div>
 
-<!-- 정보 패널: 우측 전체 높이 drawer -->
-{#if infoPanelOpen && selectedDoc}
-  <div class="fixed inset-0 z-40">
-    <button
-      onclick={() => infoPanelOpen = false}
-      class="absolute inset-0 bg-black/40"
-      aria-label="정보 패널 닫기"
-    ></button>
-    <div class="absolute right-0 top-0 bottom-0 z-50 w-[320px] shadow-xl">
-      <PreviewPanel doc={selectedDoc} onclose={() => infoPanelOpen = false} ondelete={() => { selectedDoc = null; infoPanelOpen = false; loadDocuments(); }} />
-    </div>
-  </div>
-{/if}
+<!-- < xl 폴백: Drawer primitive (우측 슬라이드) -->
+<Drawer id="meta" side="right" width="rail" aria-label="문서 정보">
+  {#if selectedDoc}
+    <DocumentMetaRail
+      doc={selectedDoc}
+      onclose={() => ui.closeDrawer()}
+      ondelete={handleDocDelete}
+    />
+  {/if}
+</Drawer>