feat: add frontend support for dual-track processing

- Add ProcessingTrack, ProcessingMetadata types to apiV2.ts
- Add analyzeDocument, getProcessingMetadata, downloadUnified API methods
- Update startTask to support ProcessingOptions
- Update TaskDetailPage with:
  - Processing track badge and description display
  - Enhanced stats grid (pages, text regions, tables, images, confidence)
  - UnifiedDocument download option
  - Translation UI preparation (disabled, awaiting backend)
- Mark Section 7 Frontend Updates as completed in tasks.md

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

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

frontend/src/services/apiV2.ts

@@ -30,6 +30,9 @@ import type {
   AuditLog,
   AuditLogListResponse,
   UserActivitySummary,
+  ProcessingOptions,
+  ProcessingMetadata,
+  DocumentAnalysisResponse,
 } from '@/types/apiV2'
 
 /**
@@ -385,10 +388,32 @@ class ApiClientV2 {
   }
 
   /**
-   * Start task processing
+   * Start task processing with optional dual-track settings
    */
-  async startTask(taskId: string): Promise<Task> {
-    const response = await this.client.post<Task>(`/tasks/${taskId}/start`)
+  async startTask(taskId: string, options?: ProcessingOptions): Promise<Task> {
+    const params = options ? {
+      use_dual_track: options.use_dual_track ?? true,
+      force_track: options.force_track,
+      language: options.language ?? 'ch',
+    } : {}
+
+    const response = await this.client.post<Task>(`/tasks/${taskId}/start`, null, { params })
+    return response.data
+  }
+
+  /**
+   * Analyze document to get recommended processing track
+   */
+  async analyzeDocument(taskId: string): Promise<DocumentAnalysisResponse> {
+    const response = await this.client.get<DocumentAnalysisResponse>(`/tasks/${taskId}/analyze`)
+    return response.data
+  }
+
+  /**
+   * Get processing metadata for a completed task
+   */
+  async getProcessingMetadata(taskId: string): Promise<ProcessingMetadata> {
+    const response = await this.client.get<ProcessingMetadata>(`/tasks/${taskId}/metadata`)
     return response.data
   }
 
@@ -475,6 +500,22 @@ class ApiClientV2 {
     window.URL.revokeObjectURL(link.href)
   }
 
+  /**
+   * Download task result as UnifiedDocument JSON
+   */
+  async downloadUnified(taskId: string): Promise<void> {
+    const response = await this.client.get(`/tasks/${taskId}/download/unified`, {
+      responseType: 'blob',
+    })
+
+    const blob = new Blob([response.data], { type: 'application/json' })
+    const link = document.createElement('a')
+    link.href = window.URL.createObjectURL(blob)
+    link.download = `${taskId}_unified.json`
+    link.click()
+    window.URL.revokeObjectURL(link.href)
+  }
+
   // ==================== Admin APIs ====================
 
   /**