Implement optimistic UI for file uploads and enhance message handling (#967)

* feat(upload): implement optimistic UI for file uploads and enhance message handling

* feat(middleware): enhance file handling by collecting historical uploads from directory

* feat(thread-title): update page title handling for new threads and improve loading state

* feat(uploads-middleware): enhance file extraction by verifying file existence in uploads directory

* feat(thread-stream): update file path reference to use virtual_path for uploads

* feat(tests): add core behaviour tests for UploadsMiddleware

* feat(tests): remove unused pytest import from test_uploads_middleware_core_logic.py

* feat: enhance file upload handling and localization support

- Update UploadsMiddleware to validate filenames more robustly.
- Modify MessageListItem to parse uploaded files from raw content for backward compatibility.
- Add localization for uploading messages in English and Chinese.
- Introduce parseUploadedFiles utility to extract uploaded files from message content.

frontend/src/core/i18n/locales/zh-CN.ts

@@ -238,6 +238,11 @@ export const zhCN: Translations = {
     skillInstallTooltip: "安装技能并使其可在 DeerFlow 中使用",
   },
 
+  uploads: {
+    uploading: "上传中...",
+    uploadingFiles: "文件上传中，请稍候...",
+  },
+
   subtasks: {
     subtask: "子任务",
     executing: (count: number) =>