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/en-US.ts

@@ -88,9 +88,11 @@ export const enUS: Translations = {
     reasoningEffortLow: "Low",
     reasoningEffortLowDescription: "Simple Logic Check + Shallow Deduction",
     reasoningEffortMedium: "Medium",
-    reasoningEffortMediumDescription: "Multi-layer Logic Analysis + Basic Verification",
+    reasoningEffortMediumDescription:
+      "Multi-layer Logic Analysis + Basic Verification",
     reasoningEffortHigh: "High",
-    reasoningEffortHighDescription: "Full-dimensional Logic Deduction + Multi-path Verification + Backward Check",
+    reasoningEffortHighDescription:
+      "Full-dimensional Logic Deduction + Multi-path Verification + Backward Check",
     searchModels: "Search models...",
     surpriseMe: "Surprise",
     surpriseMePrompt: "Surprise me",
@@ -248,6 +250,11 @@ export const enUS: Translations = {
   },
 
   // Subtasks
+  uploads: {
+    uploading: "Uploading...",
+    uploadingFiles: "Uploading files, please wait...",
+  },
+
   subtasks: {
     subtask: "Subtask",
     executing: (count: number) =>

frontend/src/core/i18n/locales/types.ts

@@ -187,6 +187,12 @@ export interface Translations {
     skillInstallTooltip: string;
   };
 
+  // Uploads
+  uploads: {
+    uploading: string;
+    uploadingFiles: string;
+  };
+
   // Subtasks
   subtasks: {
     subtask: string;

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) =>