feat: unify Direct Track PDF rendering and simplify export options

Backend changes:
- Apply background image + invisible text layer to all Direct Track PDFs
- Add CHART to regions_to_avoid for text extraction
- Improve visual fidelity for native PDFs and Office documents

Frontend changes:
- Remove JSON, UnifiedDocument, Markdown download buttons
- Simplify to 2-column layout with only Layout PDF and Reflow PDF
- Remove translation JSON download and Layout PDF option
- Keep only Reflow PDF for translated document downloads
- Clean up unused imports (FileJson, Database, FileOutput)

Archives two OpenSpec proposals:
- unify-direct-track-pdf-rendering
- simplify-frontend-export-options

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

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

openspec/changes/archive/2025-12-11-simplify-frontend-export-options/proposal.md

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+# Change: Simplify Frontend Export Options
+
+## Why
+
+The current frontend has accumulated export options that are no longer needed or rarely used. Following the "Simple OCR" architecture change, we need to streamline the user interface by:
+
+1. Removing redundant export formats that add complexity without significant user value
+2. Focusing on the most useful output formats (PDF)
+3. Simplifying the translation download options
+
+## What Changes
+
+### TaskDetailPage Changes
+
+**Download Options - Remove:**
+- JSON download button
+- UnifiedDocument (統一格式) download button
+- Markdown download button
+
+**Download Options - Keep:**
+- 版面 PDF (Layout PDF)
+- 流式 PDF (Reflow PDF)
+
+**Translation Options - Remove:**
+- Download translation JSON button
+- Download translated Layout PDF option
+
+**Translation Options - Keep:**
+- Download translated Reflow PDF (流式 PDF)
+
+**Statistics Section - Keep All:**
+- 處理時間 (Processing time)
+- 頁數 (Page count)
+- 文本區域 (Text regions)
+- 表格 (Tables)
+- 圖片 (Images)
+- 平均置信度 (Average confidence)
+
+### Components - Keep All
+- LayoutModelSelector
+- PreprocessingSettings
+- PreprocessingPreview
+- ProcessingTrackSelector
+
+### Pages to Review (Out of Scope)
+- SettingsPage (Export rules) - May need separate review
+- ResultsPage - May be unused, needs verification
+
+## Impact
+
+- **Affected files**: `frontend/src/pages/TaskDetailPage.tsx`
+- **User experience**: Simplified interface with fewer but more relevant options
+- **Backend**: No changes required (endpoints remain available for API users)
+
+## Migration
+
+- No data migration required
+- Frontend-only changes
+- Backend endpoints remain unchanged for API compatibility