feat: Markdown 뷰어/편집기 개선

- startEdit(): extracted_text || rawMarkdown fallback
- split editor → 편집/미리보기 탭 전환 방식
- GitHub Dark 스타일 markdown-body CSS (테이블/코드/인용/리스트)
- prose 클래스 → markdown-body로 교체

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

frontend/src/lib/components/DocumentViewer.svelte

@@ -69,10 +69,13 @@
   }
 
   function startEdit() {
-    editContent = fullDoc?.extracted_text || '';
+    editContent = fullDoc?.extracted_text || rawMarkdown || '';
     editMode = true;
+    editTab = 'edit';
   }
 
+  let editTab = $state('edit'); // 'edit' | 'preview'
+
   async function saveContent() {
     saving = true;
     try {
@@ -155,19 +158,32 @@
     {:else if fullDoc}
       {#if viewerType === 'markdown'}
         {#if editMode}
-          <!-- Markdown split editor -->
-          <div class="flex h-full">
-            <textarea
-              bind:value={editContent}
-              class="w-1/2 h-full p-4 bg-[var(--bg)] text-[var(--text)] text-sm font-mono resize-none outline-none border-r border-[var(--border)]"
-              spellcheck="false"
-            ></textarea>
-            <div class="w-1/2 h-full p-4 overflow-auto prose prose-invert prose-sm max-w-none">
-              {@html marked(editContent)}
+          <!-- Markdown 편집 (탭 전환) -->
+          <div class="flex flex-col h-full">
+            <div class="flex gap-1 px-3 py-1 border-b border-[var(--border)] shrink-0">
+              <button
+                onclick={() => editTab = 'edit'}
+                class="px-3 py-1 text-xs rounded-t {editTab === 'edit' ? 'bg-[var(--surface)] text-[var(--text)]' : 'text-[var(--text-dim)]'}"
+              >편집</button>
+              <button
+                onclick={() => editTab = 'preview'}
+                class="px-3 py-1 text-xs rounded-t {editTab === 'preview' ? 'bg-[var(--surface)] text-[var(--text)]' : 'text-[var(--text-dim)]'}"
+              >미리보기</button>
             </div>
+            {#if editTab === 'edit'}
+              <textarea
+                bind:value={editContent}
+                class="flex-1 w-full p-4 bg-[var(--bg)] text-[var(--text)] text-sm font-mono resize-none outline-none"
+                spellcheck="false"
+              ></textarea>
+            {:else}
+              <div class="flex-1 overflow-auto p-4 markdown-body">
+                {@html marked(editContent)}
+              </div>
+            {/if}
           </div>
         {:else}
-          <div class="p-4 prose prose-invert prose-sm max-w-none">
+          <div class="p-4 markdown-body">
             {@html marked(fullDoc.extracted_text || rawMarkdown || '*텍스트 추출 대기 중*')}
           </div>
         {/if}