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/inbox/+page.svelte

@@ -97,12 +97,10 @@
   ];
 </script>
 
-<div class="min-h-screen">
-  <nav class="flex items-center justify-between px-6 py-3 border-b border-[var(--border)] bg-[var(--surface)]">
-    <div class="flex items-center gap-4">
-      <a href="/" class="text-lg font-semibold hover:text-[var(--accent)]">PKM</a>
-      <span class="text-[var(--text-dim)]">/</span>
-      <span>Inbox</span>
+<div class="p-4 lg:p-6">
+  <div class="flex items-center justify-between mb-4">
+    <div class="flex items-center gap-3">
+      <h2 class="text-lg font-semibold">Inbox</h2>
       <span class="text-xs px-2 py-0.5 rounded-full bg-[var(--warning)] text-black">{documents.length}</span>
     </div>
     <div class="flex items-center gap-2">
@@ -117,7 +115,7 @@
         {approving ? '처리 중...' : `선택 승인 (${selected.size})`}
       </button>
     </div>
-  </nav>
+  </div>
 
   <div class="max-w-6xl mx-auto p-6">
     {#if loading}