Add batch delete functionality and enhanced pagination to conversation management interface

[Copilot]

This PR implements batch delete functionality and enhanced pagination for the conversation management interface, addressing user feedback about the inconvenience of deleting conversations one by one and improving navigation through conversation data.

Changes Made

Backend Implementation

• Added /conversation/bulk_delete endpoint to handle batch deletion requests
• Implemented comprehensive error handling with detailed success/failure reporting
• Reused existing conversation_manager.delete_conversation() logic for consistency
• Added proper validation for request parameters and conversation data

Frontend Implementation

• Added checkbox selection column to the conversation data table
• Implemented "select all/none" functionality with indeterminate state support
• Added batch action toolbar that appears when conversations are selected
• Created bulk delete confirmation dialog with preview of selected conversations
• Integrated proper state management for selection cleanup and error handling

Enhanced Pagination System

• Fixed conflicting v-data-table pagination configuration that was causing display issues
• Added comprehensive pagination information display (total records, current page status)
• Implemented inline page size selector with options for 10, 20, 50, or 100 items per page
• Enhanced v-pagination component with first/last page navigation and improved visibility
• Added proper conditional rendering for pagination controls (only shows when data exists)
• Improved visual hierarchy with informative chips showing "共 X 条记录" and "第 X / Y 页"

User Experience Improvements

The new interface allows users to:

• Select individual conversations using checkboxes
• Select/deselect all conversations using the header checkbox
• View selected conversation count in the action toolbar
• Delete multiple conversations simultaneously with a single confirmation
• See detailed success/failure results for batch operations
• Navigate through conversation data with enhanced pagination controls
• Change page size dynamically without losing current context
• View comprehensive pagination status at a glance

Technical Details

The implementation follows a minimal-change approach, preserving all existing functionality while adding the requested batch and pagination capabilities. The bulk delete operation processes each conversation individually and provides detailed feedback about any failures, ensuring robust error handling even with partial operation success. The pagination system now properly handles all edge cases and provides a more intuitive user experience.

Fixes #2143.

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[Soulter]

@copilot 现在前端的 ConversationPage 没有 paginators，请顺便也加上。

[Copilot]

@copilot 现在前端的 ConversationPage 没有 paginators，请顺便也加上。

已添加增强的分页功能 (commit b3becc7)：

✅ 修复了 v-data-table 配置问题（移除冲突的硬编码 items-per-page="10"）
✅ 添加了完整的分页信息显示（总记录数、当前页数）
✅ 添加了内联页面大小选择器（每页 10/20/50/100 条）
✅ 增强了分页控件（显示首页/末页按钮，更好的可见性）
✅ 改进了布局和用户体验

分页控件现在更加显著和用户友好，提供了完整的数据导航控制。