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/pages/TaskDetailPage.tsx

@@ -25,7 +25,8 @@ import {
   Languages,
   Globe,
   CheckCircle,
-  Trash2
+  Trash2,
+  FileOutput
 } from 'lucide-react'
 import type { ProcessingTrack, TranslationStatus, TranslationListItem } from '@/types/apiV2'
 import { Badge } from '@/components/ui/badge'
@@ -327,6 +328,24 @@ export default function TaskDetailPage() {
     }
   }
 
+  const handleDownloadTranslatedPdf = async (lang: string) => {
+    if (!taskId) return
+    try {
+      await apiClientV2.downloadTranslatedPdf(taskId, lang)
+      toast({
+        title: '下載成功',
+        description: `翻譯 PDF (${lang}) 已下載`,
+        variant: 'success',
+      })
+    } catch (error: any) {
+      toast({
+        title: '下載失敗',
+        description: error.response?.data?.detail || t('errors.networkError'),
+        variant: 'destructive',
+      })
+    }
+  }
+
   const getStatusBadge = (status: string) => {
     switch (status) {
       case 'completed':
@@ -603,7 +622,16 @@ export default function TaskDetailPage() {
                           className="gap-1"
                         >
                           <Download className="w-3 h-3" />
-                          下載
+                          JSON
+                        </Button>
+                        <Button
+                          variant="default"
+                          size="sm"
+                          onClick={() => handleDownloadTranslatedPdf(item.target_lang)}
+                          className="gap-1"
+                        >
+                          <FileOutput className="w-3 h-3" />
+                          PDF
                         </Button>
                         <Button
                           variant="ghost"