feat: optimize task file generation and add visualization download

Backend changes:
- Disable PP-Structure debug file generation by default
- Separate raw_ocr_regions.json generation from debug flag (critical file)
- Add visualization folder download endpoint as ZIP
- Add has_visualization field to TaskDetailResponse
- Stop generating Markdown files
- Save translated PDFs to task folder with caching

Frontend changes:
- Replace JSON/MD download buttons with PDF buttons in TaskHistoryPage
- Add visualization download button in TaskDetailPage
- Fix Processing page task switching issue (reset isNotFound)

Archives two OpenSpec proposals:
- optimize-task-files-and-visualization
- simplify-frontend-add-billing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/TaskDetailPage.tsx

@@ -300,6 +300,24 @@ export default function TaskDetailPage() {
     }
   }
 
+  const handleDownloadVisualization = async () => {
+    if (!taskId) return
+    try {
+      await apiClientV2.downloadVisualization(taskId)
+      toast({
+        title: '下載成功',
+        description: '辨識結果圖片已下載',
+        variant: 'success',
+      })
+    } catch (error: any) {
+      toast({
+        title: '下載失敗',
+        description: error.response?.data?.detail || t('errors.networkError'),
+        variant: 'destructive',
+      })
+    }
+  }
+
   const getStatusBadge = (status: string) => {
     switch (status) {
       case 'completed':
@@ -477,6 +495,19 @@ export default function TaskDetailPage() {
                 <span>流式 PDF</span>
               </Button>
             </div>
+            {/* Visualization download for OCR Track */}
+            {taskDetail?.has_visualization && (
+              <div className="mt-3 pt-3 border-t">
+                <Button
+                  onClick={handleDownloadVisualization}
+                  variant="secondary"
+                  className="w-full gap-2"
+                >
+                  <Image className="w-4 h-4" />
+                  下載辨識結果圖片 (ZIP)
+                </Button>
+              </div>
+            )}
           </CardContent>
         </Card>
       )}

frontend/src/pages/TaskHistoryPage.tsx

@@ -155,17 +155,11 @@ export default function TaskHistoryPage() {
   }
 
   // Download handlers
-  const handleDownload = async (taskId: string, format: 'json' | 'markdown' | 'pdf') => {
+  const handleDownloadPDF = async (taskId: string, format: 'layout' | 'reflow') => {
     try {
-      if (format === 'json') {
-        await apiClientV2.downloadJSON(taskId)
-      } else if (format === 'markdown') {
-        await apiClientV2.downloadMarkdown(taskId)
-      } else if (format === 'pdf') {
-        await apiClientV2.downloadPDF(taskId)
-      }
+      await apiClientV2.downloadPDF(taskId, format)
     } catch (err: any) {
-      alert(err.response?.data?.detail || `下載 ${format.toUpperCase()} 檔案失敗`)
+      alert(err.response?.data?.detail || `下載 PDF 檔案失敗`)
     }
   }
 
@@ -509,39 +503,24 @@ export default function TaskHistoryPage() {
                           {/* Download actions for completed tasks */}
                           {task.status === 'completed' && (
                             <>
-                              {task.result_json_path && (
-                                <Button
-                                  variant="outline"
-                                  size="sm"
-                                  onClick={() => handleDownload(task.task_id, 'json')}
-                                  title="下載 JSON"
-                                >
-                                  <Download className="w-3 h-3 mr-1" />
-                                  JSON
-                                </Button>
-                              )}
-                              {task.result_markdown_path && (
-                                <Button
-                                  variant="outline"
-                                  size="sm"
-                                  onClick={() => handleDownload(task.task_id, 'markdown')}
-                                  title="下載 Markdown"
-                                >
-                                  <Download className="w-3 h-3 mr-1" />
-                                  MD
-                                </Button>
-                              )}
-                              {task.result_pdf_path && (
-                                <Button
-                                  variant="outline"
-                                  size="sm"
-                                  onClick={() => handleDownload(task.task_id, 'pdf')}
-                                  title="下載 PDF"
-                                >
-                                  <Download className="w-3 h-3 mr-1" />
-                                  PDF
-                                </Button>
-                              )}
+                              <Button
+                                variant="outline"
+                                size="sm"
+                                onClick={() => handleDownloadPDF(task.task_id, 'layout')}
+                                title="下載版面 PDF"
+                              >
+                                <Download className="w-3 h-3 mr-1" />
+                                版面
+                              </Button>
+                              <Button
+                                variant="outline"
+                                size="sm"
+                                onClick={() => handleDownloadPDF(task.task_id, 'reflow')}
+                                title="下載流式 PDF"
+                              >
+                                <Download className="w-3 h-3 mr-1" />
+                                流式
+                              </Button>
                               <Button
                                 variant="outline"
                                 size="sm"