feat: add translated PDF export with layout preservation

Adds the ability to download translated documents as PDF files while
preserving the original document layout. Key changes:

- Add apply_translations() function to merge translation JSON with UnifiedDocument
- Add generate_translated_pdf() method to PDFGeneratorService
- Add POST /api/v2/translate/{task_id}/pdf endpoint
- Add downloadTranslatedPdf() method and PDF button in frontend
- Add comprehensive unit tests (52 tests: merge, PDF generation, API endpoints)
- Archive add-translated-pdf-export proposal

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

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

openspec/specs/result-export/spec.md

@@ -168,3 +168,57 @@ The system SHALL support exporting translation results as independent JSON files
 - **THEN** system SHALL return list of available translation languages
 - **AND** include translation metadata (translated_at, provider, statistics)
 
+### Requirement: Translated PDF Export API
+
+The system SHALL expose an API endpoint for downloading translated documents as PDF files.
+
+#### Scenario: Download translated PDF via API
+- **GIVEN** a task with completed translation to English
+- **WHEN** POST request to `/api/v2/translate/{task_id}/pdf?lang=en`
+- **THEN** system returns PDF file with translated content
+- **AND** Content-Type is `application/pdf`
+- **AND** Content-Disposition suggests filename like `{task_id}_translated_en.pdf`
+
+#### Scenario: Download translated PDF with layout preservation
+- **WHEN** user downloads translated PDF
+- **THEN** the PDF maintains original document layout
+- **AND** text positions match original document coordinates
+- **AND** images and tables appear at original positions
+
+#### Scenario: Invalid language parameter
+- **GIVEN** a task with translation only to English
+- **WHEN** user requests PDF with `lang=ja` (Japanese)
+- **THEN** system returns 404 Not Found
+- **AND** response includes available languages in error message
+
+#### Scenario: Task not found
+- **GIVEN** non-existent task_id
+- **WHEN** user requests translated PDF
+- **THEN** system returns 404 Not Found
+
+---
+
+### Requirement: Frontend Translated PDF Download
+
+The frontend SHALL provide UI controls for downloading translated PDFs.
+
+#### Scenario: Show download button when translation complete
+- **GIVEN** a task with translation status "completed"
+- **WHEN** user views TaskDetailPage
+- **THEN** page displays "Download Translated PDF" button
+- **AND** button shows target language (e.g., "Download Translated PDF (English)")
+
+#### Scenario: Hide download button when no translation
+- **GIVEN** a task without any completed translations
+- **WHEN** user views TaskDetailPage
+- **THEN** "Download Translated PDF" button is not shown
+
+#### Scenario: Download progress indication
+- **GIVEN** user clicks "Download Translated PDF" button
+- **WHEN** PDF generation is in progress
+- **THEN** button shows loading state
+- **AND** prevents double-click
+- **WHEN** download completes
+- **THEN** browser downloads PDF file
+- **AND** button returns to normal state
+