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>

backend/app/core/config.py

@@ -376,8 +376,8 @@ class Settings(BaseSettings):
 
     # ===== Debug Configuration =====
     # Enable debug outputs for PP-StructureV3 analysis
-    pp_structure_debug_enabled: bool = Field(default=True)  # Save debug files for PP-StructureV3
-    pp_structure_debug_visualization: bool = Field(default=True)  # Generate visualization images
+    pp_structure_debug_enabled: bool = Field(default=False)  # Save debug files for PP-StructureV3
+    pp_structure_debug_visualization: bool = Field(default=False)  # Generate visualization images
 
     # Performance tuning
     use_fp16_inference: bool = Field(default=False)  # Half-precision (if supported)