feat: complete i18n support for all frontend pages and components

Add comprehensive bilingual (zh-TW/en-US) support across the entire frontend:

Pages updated:
- AdminDashboardPage: All 63+ strings translated
- TaskHistoryPage: All 80+ strings translated
- TaskDetailPage: All 90+ strings translated
- AuditLogsPage: All audit log UI translated
- ResultsPage/ProcessingPage: Fixed i18n integration
- UploadPage: Step indicators and file list UI translated

Components updated:
- TaskNotFound: Task deletion messages
- FileUpload: Prompts and file size limits
- ProcessingTrackSelector: Processing mode options with analysis info
- Layout: Navigation descriptions
- ProtectedRoute: Loading and access denied messages
- PDFViewer: Page navigation and error messages

Locale files: Added ~200 new translation keys to both zh-TW.json and en-US.json

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

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

frontend/src/pages/ProcessingPage.tsx

@@ -124,8 +124,8 @@ function SingleTaskProcessing() {
         updateTaskStatus(taskId, 'processing', forceTrack || undefined)
       }
       toast({
-        title: '開始處理',
-        description: 'OCR 處理已開始',
+        title: t('processing.startProcessing'),
+        description: t('processing.ocrStarted'),
         variant: 'success',
       })
     },
@@ -200,7 +200,7 @@ function SingleTaskProcessing() {
       <div className="flex items-center justify-center min-h-[60vh]">
         <div className="text-center">
           <Loader2 className="w-12 h-12 animate-spin text-primary mx-auto mb-4" />
-          <p className="text-muted-foreground">載入任務資訊...</p>
+          <p className="text-muted-foreground">{t('processing.loadingTask')}</p>
         </div>
       </div>
     )
@@ -226,13 +226,13 @@ function SingleTaskProcessing() {
           </CardHeader>
           <CardContent className="space-y-4">
             <p className="text-muted-foreground">
-              {t('processing.noBatchMessage', { defaultValue: '尚未選擇任何任務。請先上傳檔案以建立任務。' })}
+              {t('processing.noBatchMessage')}
             </p>
             <Button
               onClick={() => navigate('/upload')}
               size="lg"
             >
-              {t('processing.goToUpload', { defaultValue: '前往上傳頁面' })}
+              {t('processing.goToUpload')}
             </Button>
           </CardContent>
         </Card>
@@ -252,7 +252,7 @@ function SingleTaskProcessing() {
           <div>
             <h1 className="page-title">{t('processing.title')}</h1>
             <p className="text-muted-foreground mt-1">
-              任務 ID: <span className="font-mono text-primary">{taskId}</span>
+              {t('taskDetail.taskId', { id: taskId })}
               {taskDetail?.filename && ` · ${taskDetail.filename}`}
             </p>
           </div>
@@ -260,13 +260,13 @@ function SingleTaskProcessing() {
             {isCompleted && (
               <div className="flex items-center gap-2 text-success">
                 <CheckCircle className="w-6 h-6" />
-                <span className="font-semibold">處理完成</span>
+                <span className="font-semibold">{t('processing.completed')}</span>
               </div>
             )}
             {isProcessing && (
               <div className="flex items-center gap-2 text-primary">
                 <Loader2 className="w-6 h-6 animate-spin" />
-                <span className="font-semibold">處理中</span>
+                <span className="font-semibold">{t('processing.processing')}</span>
               </div>
             )}
           </div>
@@ -311,9 +311,9 @@ function SingleTaskProcessing() {
                     <FileText className="w-5 h-5 text-primary" />
                   </div>
                   <div>
-                    <p className="text-xs text-muted-foreground mb-0.5">檔案名稱</p>
+                    <p className="text-xs text-muted-foreground mb-0.5">{t('taskDetail.filename')}</p>
                     <p className="text-sm font-medium text-foreground truncate">
-                      {taskDetail.filename || '未知檔案'}
+                      {taskDetail.filename || t('common.unknownFile')}
                     </p>
                   </div>
                 </div>
@@ -325,7 +325,7 @@ function SingleTaskProcessing() {
                       <Clock className="w-5 h-5 text-success" />
                     </div>
                     <div>
-                      <p className="text-xs text-muted-foreground mb-0.5">處理時間</p>
+                      <p className="text-xs text-muted-foreground mb-0.5">{t('taskDetail.processingTime')}</p>
                       <p className="text-sm font-medium text-foreground">
                         {(taskDetail.processing_time_ms / 1000).toFixed(2)}s
                       </p>
@@ -342,7 +342,7 @@ function SingleTaskProcessing() {
               <div className="flex items-start gap-3">
                 <AlertCircle className="w-5 h-5 text-destructive flex-shrink-0 mt-0.5" />
                 <div>
-                  <p className="text-sm font-medium text-destructive mb-1">處理失敗</p>
+                  <p className="text-sm font-medium text-destructive mb-1">{t('processing.failed')}</p>
                   <p className="text-sm text-destructive/80">{taskDetail.error_message}</p>
                 </div>
               </div>
@@ -380,7 +380,7 @@ function SingleTaskProcessing() {
                   size="lg"
                 >
                   <CheckCircle className="w-4 h-4" />
-                  查看任務歷史
+                  {t('results.viewTaskHistory')}
                 </Button>
               )}
             </div>
@@ -396,24 +396,24 @@ function SingleTaskProcessing() {
               <div className="p-2 bg-primary/10 rounded-lg">
                 <FileText className="w-5 h-5 text-primary" />
               </div>
-              <CardTitle>任務詳情</CardTitle>
+              <CardTitle>{t('taskDetail.title')}</CardTitle>
             </div>
           </CardHeader>
           <CardContent>
             <div className="space-y-3">
               <div className="flex justify-between py-2 border-b border-border">
-                <span className="text-sm text-muted-foreground">任務狀態</span>
+                <span className="text-sm text-muted-foreground">{t('taskDetail.taskStatus')}</span>
                 {getStatusBadge(taskDetail.status)}
               </div>
               <div className="flex justify-between py-2 border-b border-border">
-                <span className="text-sm text-muted-foreground">建立時間</span>
+                <span className="text-sm text-muted-foreground">{t('taskDetail.createdAt')}</span>
                 <span className="text-sm font-medium">
                   {new Date(taskDetail.created_at).toLocaleString('zh-TW')}
                 </span>
               </div>
               {taskDetail.updated_at && (
                 <div className="flex justify-between py-2 border-b border-border">
-                  <span className="text-sm text-muted-foreground">更新時間</span>
+                  <span className="text-sm text-muted-foreground">{t('taskDetail.lastUpdated')}</span>
                   <span className="text-sm font-medium">
                     {new Date(taskDetail.updated_at).toLocaleString('zh-TW')}
                   </span>
@@ -421,7 +421,7 @@ function SingleTaskProcessing() {
               )}
               {taskDetail.completed_at && (
                 <div className="flex justify-between py-2 border-b border-border">
-                  <span className="text-sm text-muted-foreground">完成時間</span>
+                  <span className="text-sm text-muted-foreground">{t('taskDetail.completedAt')}</span>
                   <span className="text-sm font-medium">
                     {new Date(taskDetail.completed_at).toLocaleString('zh-TW')}
                   </span>
@@ -439,7 +439,7 @@ function SingleTaskProcessing() {
           {isAnalyzing && (
             <div className="flex items-center gap-2 p-4 bg-muted/30 rounded-lg border">
               <Loader2 className="w-4 h-4 animate-spin text-primary" />
-              <span className="text-sm text-muted-foreground">分析文件類型中...</span>
+              <span className="text-sm text-muted-foreground">{t('processing.analyzingDocument')}</span>
             </div>
           )}