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/zh.ts

@@ -826,7 +826,9 @@ General：实体和关系提取提示来自 GitHub - microsoft/graphrag：基于
       avatarHidden: '隐藏头像',
       locale: '地区',
       tocEnhance: '目录增强',
-      tocEnhanceTip: `解析文档时生成了目录信息（见General方法的‘启用目录抽取’），让大模型返回和用户问题相关的目录项，从而利用目录项拿到相关chunk，对这些chunk在排序中进行加权。这种方法来源于模仿人类查询书本中知识的行为逻辑`,
+      tocEnhanceTip: `解析文档时生成了目录信息（见General方法的'启用目录抽取'），让大模型返回和用户问题相关的目录项，从而利用目录项拿到相关chunk，对这些chunk在排序中进行加权。这种方法来源于模仿人类查询书本中知识的行为逻辑`,
+      batchDeleteSessions: '批量删除',
+      deleteSelectedConfirm: '删除选中的 {count} 个会话？',
     },
     setting: {
       deleteModel: '删除模型',