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>

frontend/src/services/apiV2.ts

@@ -686,6 +686,23 @@ class ApiClientV2 {
   async deleteTranslation(taskId: string, lang: string): Promise<void> {
     await this.client.delete(`/translate/${taskId}/translations/${lang}`)
   }
+
+  /**
+   * Download translated PDF with layout preservation
+   */
+  async downloadTranslatedPdf(taskId: string, lang: string): Promise<void> {
+    const response = await this.client.post(`/translate/${taskId}/pdf`, null, {
+      params: { lang },
+      responseType: 'blob',
+    })
+
+    const blob = new Blob([response.data], { type: 'application/pdf' })
+    const link = document.createElement('a')
+    link.href = window.URL.createObjectURL(blob)
+    link.download = `${taskId}_translated_${lang}.pdf`
+    link.click()
+    window.URL.revokeObjectURL(link.href)
+  }
 }
 
 // Export singleton instance