refactor: 사이드바를 전역 레이아웃으로 이동

- +layout.svelte: 사이드바 + 상단 nav 통합 (로그인/셋업 제외)
- 각 페이지 중복 nav 제거 (dashboard, documents, detail, inbox, settings)
- 모바일 drawer + ESC 닫기 전역 처리

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -33,14 +33,13 @@
   }
 </script>
 
-<div class="min-h-screen">
-  <nav class="flex items-center gap-2 px-6 py-3 border-b border-[var(--border)] bg-[var(--surface)] text-sm">
-    <a href="/" class="text-[var(--text-dim)] hover:text-[var(--text)]">PKM</a>
-    <span class="text-[var(--text-dim)]">/</span>
-    <a href="/documents" class="text-[var(--text-dim)] hover:text-[var(--text)]">문서</a>
-    <span class="text-[var(--text-dim)]">/</span>
-    <span class="truncate max-w-md">{doc?.title || '로딩...'}</span>
-  </nav>
+<div class="p-4 lg:p-6">
+  <!-- breadcrumb -->
+  <div class="flex items-center gap-2 text-sm mb-4 text-[var(--text-dim)]">
+    <a href="/documents" class="hover:text-[var(--text)]">문서</a>
+    <span>/</span>
+    <span class="truncate max-w-md text-[var(--text)]">{doc?.title || '로딩...'}</span>
+  </div>
 
   {#if loading}
     <div class="max-w-6xl mx-auto p-6">