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>

frontend/src/pages/TaskDetailPage.tsx

@@ -14,7 +14,6 @@ import {
   AlertCircle,
   Clock,
   Layers,
-  FileJson,
   Loader2,
   ArrowLeft,
   RefreshCw,
@@ -22,12 +21,10 @@ import {
   Table2,
   Image,
   BarChart3,
-  Database,
   Languages,
   Globe,
   CheckCircle,
-  Trash2,
-  FileOutput
+  Trash2
 } from 'lucide-react'
 import type { ProcessingTrack, TranslationStatus, TranslationListItem } from '@/types/apiV2'
 import { Badge } from '@/components/ui/badge'
@@ -224,60 +221,6 @@ export default function TaskDetailPage() {
     }
   }
 
-  const handleDownloadMarkdown = async () => {
-    if (!taskId) return
-    try {
-      await apiClientV2.downloadMarkdown(taskId)
-      toast({
-        title: t('export.exportSuccess'),
-        description: 'Markdown 已下載',
-        variant: 'success',
-      })
-    } catch (error: any) {
-      toast({
-        title: t('export.exportError'),
-        description: error.response?.data?.detail || t('errors.networkError'),
-        variant: 'destructive',
-      })
-    }
-  }
-
-  const handleDownloadJSON = async () => {
-    if (!taskId) return
-    try {
-      await apiClientV2.downloadJSON(taskId)
-      toast({
-        title: t('export.exportSuccess'),
-        description: 'JSON 已下載',
-        variant: 'success',
-      })
-    } catch (error: any) {
-      toast({
-        title: t('export.exportError'),
-        description: error.response?.data?.detail || t('errors.networkError'),
-        variant: 'destructive',
-      })
-    }
-  }
-
-  const handleDownloadUnified = async () => {
-    if (!taskId) return
-    try {
-      await apiClientV2.downloadUnified(taskId)
-      toast({
-        title: t('export.exportSuccess'),
-        description: 'UnifiedDocument JSON 已下載',
-        variant: 'success',
-      })
-    } catch (error: any) {
-      toast({
-        title: t('export.exportError'),
-        description: error.response?.data?.detail || t('errors.networkError'),
-        variant: 'destructive',
-      })
-    }
-  }
-
   const handleStartTranslation = async () => {
     if (!taskId || isTranslating) return
 
@@ -319,24 +262,6 @@ export default function TaskDetailPage() {
     }
   }
 
-  const handleDownloadTranslation = async (lang: string) => {
-    if (!taskId) return
-    try {
-      await apiClientV2.downloadTranslation(taskId, lang)
-      toast({
-        title: '下載成功',
-        description: `翻譯結果 (${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 handleDeleteTranslation = async (lang: string) => {
     if (!taskId) return
     try {
@@ -542,19 +467,7 @@ export default function TaskDetailPage() {
             </CardTitle>
           </CardHeader>
           <CardContent>
-            <div className="grid grid-cols-2 md:grid-cols-5 gap-3">
-              <Button onClick={handleDownloadJSON} variant="outline" className="gap-2 h-20 flex-col">
-                <FileJson className="w-8 h-8" />
-                <span>JSON</span>
-              </Button>
-              <Button onClick={handleDownloadUnified} variant="outline" className="gap-2 h-20 flex-col">
-                <Database className="w-8 h-8" />
-                <span>統一格式</span>
-              </Button>
-              <Button onClick={handleDownloadMarkdown} variant="outline" className="gap-2 h-20 flex-col">
-                <FileText className="w-8 h-8" />
-                <span>Markdown</span>
-              </Button>
+            <div className="grid grid-cols-2 gap-3">
               <Button onClick={handleDownloadLayoutPDF} className="gap-2 h-20 flex-col">
                 <Download className="w-8 h-8" />
                 <span>版面 PDF</span>
@@ -650,28 +563,12 @@ export default function TaskDetailPage() {
                         <Button
                           variant="outline"
                           size="sm"
-                          onClick={() => handleDownloadTranslation(item.target_lang)}
+                          onClick={() => handleDownloadTranslatedPdf(item.target_lang, 'reflow')}
                           className="gap-1"
                         >
                           <Download className="w-3 h-3" />
-                          JSON
+                          流式 PDF
                         </Button>
-                        <Select
-                          onValueChange={(format: 'layout' | 'reflow') =>
-                            handleDownloadTranslatedPdf(item.target_lang, format)
-                          }
-                        >
-                          <SelectTrigger className="w-[100px] h-8">
-                            <div className="flex items-center gap-1">
-                              <FileOutput className="w-3 h-3" />
-                              <span className="text-xs">PDF</span>
-                            </div>
-                          </SelectTrigger>
-                          <SelectContent>
-                            <SelectItem value="layout">版面 PDF</SelectItem>
-                            <SelectItem value="reflow">流式 PDF</SelectItem>
-                          </SelectContent>
-                        </Select>
                         <Button
                           variant="ghost"
                           size="sm"