fix: migrate UploadPage to V2 API and fix logout navigation

Changes:
- Add uploadFile() method to apiClientV2 for single file uploads
- Update UploadPage to use apiClientV2 instead of apiClient
- Change upload logic to iterate files and collect task IDs
- Add navigation to /login after logout in Layout component

Fixes:
- 403 Forbidden error on file upload (token mismatch between V1/V2 APIs)
- Logout button not redirecting to login page after clearing auth

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

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

frontend/src/services/apiV2.ts

@@ -284,6 +284,24 @@ class ApiClientV2 {
     return response.data.sessions
   }
 
+  // ==================== File Upload ====================
+
+  /**
+   * Upload a file
+   */
+  async uploadFile(file: File): Promise<{ task_id: string; filename: string; file_size: number; file_type: string; status: string }> {
+    const formData = new FormData()
+    formData.append('file', file)
+
+    const response = await this.client.post('/upload', formData, {
+      headers: {
+        'Content-Type': 'multipart/form-data',
+      },
+    })
+
+    return response.data
+  }
+
   // ==================== Task Management ====================
 
   /**