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-unify-direct-track-pdf-rendering/specs/document-processing/spec.md

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+## ADDED Requirements
+
+### Requirement: Direct Track Background Image Rendering
+
+The system SHALL render Direct Track PDF output using a full-page background image with an invisible text overlay to preserve visual fidelity while maintaining text extractability.
+
+#### Scenario: Render Direct Track PDF with background image
+- **WHEN** generating Layout PDF for a Direct Track document
+- **THEN** the system SHALL render each source PDF page as a full-page background image at 2x resolution
+- **AND** overlay invisible text elements using PDF Text Rendering Mode 3
+- **AND** the invisible text SHALL be positioned at original coordinates for accurate selection
+
+#### Scenario: Handle Office documents (PPT, DOC, XLS)
+- **WHEN** processing an Office document converted to PDF
+- **THEN** the system SHALL use the same background image + invisible text approach
+- **AND** preserve all visual elements including vector graphics, gradients, and complex layouts
+- **AND** the converted PDF in result directory SHALL be used as background source
+
+#### Scenario: Handle native editable PDFs
+- **WHEN** processing a native PDF through Direct Track
+- **THEN** the system SHALL use the source PDF for background rendering
+- **AND** apply the same invisible text overlay approach
+- **AND** chart regions SHALL be excluded from the text layer
+
+### Requirement: Chart Region Text Exclusion
+
+The system SHALL exclude text elements within chart regions from the invisible text layer to prevent duplicate content and unnecessary translation.
+
+#### Scenario: Detect chart regions
+- **WHEN** classifying page elements for Direct Track
+- **THEN** the system SHALL identify elements with type CHART
+- **AND** add chart bounding boxes to regions_to_avoid list
+
+#### Scenario: Exclude chart-internal text from invisible layer
+- **WHEN** rendering invisible text layer
+- **THEN** the system SHALL skip text elements whose bounding boxes overlap with chart regions
+- **AND** chart axis labels, legends, and data labels SHALL NOT be in the invisible text layer
+- **AND** these texts remain visible in the background image
+
+#### Scenario: Chart text not available for translation
+- **WHEN** extracting text for translation from a Direct Track document
+- **THEN** chart-internal text SHALL NOT be included in translatable elements
+- **AND** this is expected behavior as chart labels typically don't require translation

openspec/changes/archive/2025-12-11-unify-direct-track-pdf-rendering/specs/result-export/spec.md

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+## MODIFIED Requirements
+
+### Requirement: Enhanced PDF Export with Layout Preservation
+
+The PDF export SHALL accurately preserve document layout from both OCR and direct extraction tracks with correct coordinate transformation and multi-page support. For Direct Track, a background image rendering approach SHALL be used for visual fidelity.
+
+#### Scenario: Export PDF from direct extraction track
+- **WHEN** exporting PDF from a direct-extraction processed document
+- **THEN** the system SHALL render source PDF pages as full-page background images at 2x resolution
+- **AND** overlay invisible text elements using PDF Text Rendering Mode 3
+- **AND** text SHALL remain selectable and searchable despite being invisible
+- **AND** visual output SHALL match source document exactly
+
+#### Scenario: Export PDF from OCR track with full structure
+- **WHEN** exporting PDF from OCR-processed document
+- **THEN** the PDF SHALL use all 23 PP-StructureV3 element types
+- **AND** render tables with proper cell boundaries
+- **AND** maintain reading order from parsing_res_list
+
+#### Scenario: Handle coordinate transformations correctly
+- **WHEN** generating PDF from UnifiedDocument
+- **THEN** system SHALL use explicit page dimensions from OCR results (not inferred from bounding boxes)
+- **AND** correctly transform Y-axis coordinates from top-left (OCR) to bottom-left (PDF/ReportLab) origin
+- **AND** prevent vertical flipping or position misalignment errors
+
+#### Scenario: Direct Track PDF file size increase
+- **WHEN** generating Layout PDF for Direct Track documents
+- **THEN** the system SHALL accept increased file size due to embedded page images
+- **AND** approximately 1-2 MB per page at 2x resolution is expected
+- **AND** this trade-off is accepted for improved visual fidelity
+
+#### Scenario: Chart elements excluded from text layer
+- **WHEN** generating Layout PDF containing charts
+- **THEN** the system SHALL NOT include chart-internal text in the invisible text layer
+- **AND** chart visuals SHALL be preserved in the background image
+- **AND** chart text SHALL NOT be available for text selection or translation

openspec/changes/archive/2025-12-11-unify-direct-track-pdf-rendering/specs/translation/spec.md

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+## ADDED Requirements
+
+### Requirement: Translation Output as Reflow PDF
+
+The system SHALL generate translated documents as reflow-layout PDFs with real visible text, separate from the Layout PDF which uses background images.
+
+#### Scenario: Generate translated PDF with reflow layout
+- **WHEN** translation is completed for a document
+- **THEN** the system SHALL generate a new PDF with translated text
+- **AND** the translated PDF SHALL use reflow layout (not background image)
+- **AND** text SHALL be real visible text, not invisible overlay
+- **AND** page breaks SHALL correspond to original document pages
+
+#### Scenario: Maintain page correspondence in translated output
+- **WHEN** generating translated PDF
+- **THEN** content from original page 1 SHALL appear in translated page 1
+- **AND** content from original page 2 SHALL appear in translated page 2
+- **AND** each page may have different content length but maintains page boundaries
+
+#### Scenario: Chart text excluded from translation
+- **WHEN** extracting text for translation from Direct Track documents
+- **THEN** text elements within chart regions SHALL NOT be included
+- **AND** chart labels, axis text, and legends SHALL remain untranslated
+- **AND** this is expected behavior documented for users
+
+### Requirement: Dual PDF Output Concept
+
+The system SHALL maintain clear separation between Layout PDF (preview) and Translated PDF (output).
+
+#### Scenario: Layout PDF for preview
+- **WHEN** user views a processed document before translation
+- **THEN** the Layout PDF SHALL be displayed
+- **AND** Layout PDF preserves exact visual appearance of source
+- **AND** text is invisible overlay for extraction purposes only
+
+#### Scenario: Translated PDF for final output
+- **WHEN** user requests translated document
+- **THEN** the Translated PDF SHALL be generated
+- **AND** Translated PDF uses reflow layout with visible translated text
+- **AND** original visual styling is not preserved (text-focused output)
+
+#### Scenario: Both PDFs available after translation
+- **WHEN** translation is completed
+- **THEN** both Layout PDF and Translated PDF SHALL be available for download
+- **AND** user can choose which version to download
+- **AND** Layout PDF remains unchanged after translation