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>
       )}