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/app.css

@@ -37,3 +37,31 @@ body {
 ::-webkit-scrollbar { width: 6px; }
 ::-webkit-scrollbar-track { background: var(--bg); }
 ::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
+
+/* Markdown 렌더링 (GitHub Dark 스타일) */
+.markdown-body {
+  color: var(--text);
+  line-height: 1.7;
+  font-size: 14px;
+}
+.markdown-body h1 { font-size: 1.6em; font-weight: 700; margin: 1.5em 0 0.5em; padding-bottom: 0.3em; border-bottom: 1px solid var(--border); }
+.markdown-body h2 { font-size: 1.3em; font-weight: 600; margin: 1.3em 0 0.4em; padding-bottom: 0.2em; border-bottom: 1px solid var(--border); }
+.markdown-body h3 { font-size: 1.1em; font-weight: 600; margin: 1.2em 0 0.3em; }
+.markdown-body h4 { font-size: 1em; font-weight: 600; margin: 1em 0 0.2em; }
+.markdown-body p { margin: 0.6em 0; }
+.markdown-body ul, .markdown-body ol { padding-left: 1.5em; margin: 0.5em 0; }
+.markdown-body li { margin: 0.2em 0; }
+.markdown-body li > ul, .markdown-body li > ol { margin: 0.1em 0; }
+.markdown-body blockquote { border-left: 3px solid var(--accent); padding: 0.5em 1em; margin: 0.8em 0; color: var(--text-dim); background: var(--surface); border-radius: 0 4px 4px 0; }
+.markdown-body code { background: var(--surface); padding: 0.15em 0.4em; border-radius: 3px; font-size: 0.9em; font-family: 'SF Mono', Menlo, monospace; }
+.markdown-body pre { background: var(--surface); padding: 1em; border-radius: 6px; overflow-x: auto; margin: 0.8em 0; border: 1px solid var(--border); }
+.markdown-body pre code { background: none; padding: 0; }
+.markdown-body table { border-collapse: collapse; width: 100%; margin: 0.8em 0; }
+.markdown-body th, .markdown-body td { border: 1px solid var(--border); padding: 0.5em 0.8em; text-align: left; font-size: 0.9em; }
+.markdown-body th { background: var(--surface); font-weight: 600; }
+.markdown-body tr:nth-child(even) { background: rgba(255,255,255,0.02); }
+.markdown-body hr { border: none; border-top: 1px solid var(--border); margin: 1.5em 0; }
+.markdown-body a { color: var(--accent); text-decoration: none; }
+.markdown-body a:hover { text-decoration: underline; }
+.markdown-body strong { font-weight: 600; }
+.markdown-body img { max-width: 100%; border-radius: 4px; }

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">
+          <!-- 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="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)]"
+                class="flex-1 w-full p-4 bg-[var(--bg)] text-[var(--text)] text-sm font-mono resize-none outline-none"
                 spellcheck="false"
               ></textarea>
-            <div class="w-1/2 h-full p-4 overflow-auto prose prose-invert prose-sm max-w-none">
+            {: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}