feat: add batch delete for conversations in chat（web） (#12584)

Resolves #12572

## What problem does this PR solve?
The conversation list in chat sessions previously only supported
deleting conversations one by one. This was inefficient when users
needed to clean up multiple conversations. This PR adds batch delete
functionality to improve user experience.

## Type of change
 - [x] New Feature (non-breaking change which adds functionality)

## Specific changes
  - Add selection mode with checkboxes for conversation list
  - Add batch delete functionality with custom icons
  - Add internationalization support (en/zh)
  - Use existing removeConversation API which supports batch deletion

## UI modification status
  - Default: Show [+] and [batch delete icon]
  - Selection mode: Show checkboxes, keep [+] and [select all icon]
  - Items selected: Show [return icon] and [red trash icon]"

### Repair Comparison
**1.Before Repair**
<img width="982" height="1221" alt="image"
src="https://github.com/user-attachments/assets/8a80f7c0-7da6-41ec-9d1a-ac887ede96ba"
/>


**2.After Repair**
<img width="1273" height="919" alt="新增批量删除效果图"
src="https://github.com/user-attachments/assets/e179bdf3-3779-4bd5-84b6-8e24780a22ea"
/>

---
Co-authored-by: Gongzi

---------

Co-authored-by: Liu An <asiro@qq.com>

web/src/locales/en.ts

@@ -427,7 +427,8 @@ Example: A 1 KB message with 1024-dim embedding uses ~9 KB. The 5 MB default lim
       paddleocrOptions: 'PaddleOCR Options',
       paddleocrApiUrl: 'PaddleOCR API URL',
       paddleocrApiUrlTip: 'The API endpoint URL for PaddleOCR service',
-      paddleocrApiUrlPlaceholder: 'e.g. https://paddleocr-server.com/layout-parsing',
+      paddleocrApiUrlPlaceholder:
+        'e.g. https://paddleocr-server.com/layout-parsing',
       paddleocrAccessToken: 'AI Studio Access Token',
       paddleocrAccessTokenTip: 'Access token for PaddleOCR API (optional)',
       paddleocrAccessTokenPlaceholder: 'Your AI Studio token (optional)',
@@ -866,6 +867,8 @@ This auto-tagging feature enhances retrieval by adding another layer of domain-s
       chatSetting: 'Chat setting',
       tocEnhance: 'TOC enhance',
       tocEnhanceTip: ` During the parsing of the document, table of contents information was generated (see the 'Enable Table of Contents Extraction' option in the General method). This allows the large model to return table of contents items relevant to the user's query, thereby using these items to retrieve related chunks and apply weighting to these chunks during the sorting process. This approach is derived from mimicking the behavioral logic of how humans search for knowledge in books.`,
+      batchDeleteSessions: 'Batch delete',
+      deleteSelectedConfirm: 'Delete the selected {count} session(s)?',
     },
     setting: {
       deleteModel: 'Delete model',
@@ -1107,14 +1110,15 @@ Example: Virtual Hosted Style`,
       baseUrlNameMessage: 'Please input your base url!',
       paddleocr: {
         apiUrl: 'PaddleOCR API URL',
-        apiUrlPlaceholder: 'For example: https://paddleocr-server.com/layout-parsing',
+        apiUrlPlaceholder:
+          'For example: https://paddleocr-server.com/layout-parsing',
         accessToken: 'AI Studio Access Token',
         accessTokenPlaceholder: 'Your AI Studio token (optional)',
         algorithm: 'PaddleOCR Algorithm',
         selectAlgorithm: 'Select Algorithm',
         modelNamePlaceholder: 'For example: paddleocr-from-env-1',
         modelNameRequired: 'Model name is required',
-        apiUrlRequired: 'PaddleOCR API URL is required'
+        apiUrlRequired: 'PaddleOCR API URL is required',
       },
       vision: 'Does it support Vision?',
       ollamaLink: 'How to integrate {{name}}',