feat(ui): 데스크탑 분류 사이드바 접기/펴기 토글

상단 nav 좌측 PanelLeft 버튼으로 좌측 분류(소스트리) 사이드바를 접고/펼침.
접으면 aside w-sidebar→w-0(+border 제거)로 콘텐츠가 넓어짐, 상태는 localStorage 기억.
확정 시안(documents-confirmed-column-browser)의 '소스트리 접기/펴기' 반영.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -3,7 +3,7 @@
   import { browser } from '$app/environment';
   import { page } from '$app/stores';
   import { goto } from '$app/navigation';
-  import { Menu, EllipsisVertical, ChevronDown, FileText, Newspaper, HelpCircle, StickyNote, Inbox } from 'lucide-svelte';
+  import { Menu, EllipsisVertical, ChevronDown, FileText, Newspaper, HelpCircle, StickyNote, Inbox, PanelLeft } from 'lucide-svelte';
   import { isAuthenticated, user, tryRefresh, logout } from '$lib/stores/auth';
   import { toasts, removeToast } from '$lib/stores/toast';
   import { refresh as refreshPublicConfig } from '$lib/stores/config';
@@ -32,6 +32,15 @@
   let menuOpen = $state(false);   // ⋮ 설정 메뉴
   let navMenu = $state('');        // '' | 'docs' | 'news' — 상단 드롭다운
 
+  // 데스크탑 분류(소스트리) 사이드바 접기/펴기 — localStorage 기억. 접으면 콘텐츠가 넓어짐.
+  let sidebarCollapsed = $state(
+    typeof localStorage !== 'undefined' ? localStorage.getItem('sidebarCollapsed') === 'true' : false
+  );
+  function toggleSidebarCollapse() {
+    sidebarCollapsed = !sidebarCollapsed;
+    if (typeof localStorage !== 'undefined') localStorage.setItem('sidebarCollapsed', String(sidebarCollapsed));
+  }
+
   function isActive(path) {
     return $page.url.pathname.startsWith(path);
   }
@@ -85,6 +94,11 @@
           <div class="lg:hidden">
             <IconButton icon={Menu} size="sm" aria-label="사이드바" onclick={() => ui.openDrawer('sidebar')} />
           </div>
+          <div class="hidden lg:block">
+            <IconButton icon={PanelLeft} size="sm" aria-label={sidebarCollapsed ? '사이드바 펴기' : '사이드바 접기'}
+              aria-pressed={!sidebarCollapsed} title={sidebarCollapsed ? '사이드바 펴기' : '사이드바 접기'}
+              onclick={toggleSidebarCollapse} />
+          </div>
         {/if}
         <a href="/" class="flex items-center gap-2 shrink-0">
           <span class="w-7 h-7 rounded-md bg-accent text-white grid place-items-center text-[10px] font-extrabold tracking-wide">DS</span>
@@ -150,7 +164,7 @@
       <!-- 메인: 데스크탑 상시 사이드바 + 콘텐츠 -->
       <div class="flex-1 min-h-0 flex">
         {#if showSidebar}
-          <aside class="hidden lg:block w-sidebar shrink-0 overflow-hidden border-r border-default">
+          <aside class="hidden lg:block shrink-0 overflow-hidden transition-[width] duration-200 ease-out {sidebarCollapsed ? 'w-0 border-r-0' : 'w-sidebar border-r border-default'}">
             <Sidebar />
           </aside>
         {/if}