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

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

openspec/changes/archive/2025-12-11-simplify-frontend-export-options/specs/result-export/spec.md

@@ -0,0 +1,24 @@
+## MODIFIED Requirements
+
+### Requirement: Export Interface
+
+The Export interface in TaskDetailPage SHALL provide streamlined download options focusing on PDF formats.
+
+#### Scenario: Download options for completed tasks
+- **WHEN** viewing a completed task in TaskDetailPage
+- **THEN** the download section SHALL display only two buttons: "版面 PDF" and "流式 PDF"
+- **AND** JSON, UnifiedDocument, and Markdown download buttons SHALL NOT be displayed
+- **AND** the download grid SHALL use a 2-column layout
+
+#### Scenario: Translation download options
+- **WHEN** viewing completed translations in TaskDetailPage
+- **THEN** each translation item SHALL display only a "流式 PDF" download button
+- **AND** translation JSON download button SHALL NOT be displayed
+- **AND** Layout PDF option for translations SHALL NOT be displayed
+- **AND** delete translation button SHALL remain available
+
+#### Scenario: Backend API remains unchanged
+- **WHEN** external clients call download endpoints directly
+- **THEN** JSON, Markdown, and UnifiedDocument endpoints SHALL still function
+- **AND** translated Layout PDF endpoint SHALL still function
+- **AND** no backend changes are required for this frontend simplification