[sight] feat: add optimization tips and savings breakdown to token savings page

[husterL9]

Component

sight

Problem Statement

Token Savings 页面缺乏可操作性：

1. 用户无法直观了解 tokenless 配置是否最优，缺少针对性优化建议
2. 缺少跨会话维度的节省排行，难以定位最大收益来源
3. 单条优化记录无压缩率和中文解释，可读性差

Proposed Solution

后端 (handlers.rs)：

• OptimizationItemDto 新增 compression_ratio 和 explanation 字段
• 新增 OptimizationTip 结构体，根据节省率/组件状态自动生成多级建议（success/info/warning）

前端 (TokenSavingsPage.tsx + apiClient.ts)：

• 新增 OptimizationTipsPanel：按级别展示优化建议
• 新增 SavingsBreakdownPanel：Top 5 节省排行进度条
• DiffView 增强：explanation banner + token 数量标注

测试：新增 2 个用例覆盖新面板渲染

Alternatives Considered

• 前端硬编码建议：不够灵活，无法动态适配数据
• 单独 API 端点：增加请求次数，不如一次返回高效

Additional Context

变更文件：handlers.rs、apiClient.ts、TokenSavingsPage.tsx、TokenSavingsPage.test.tsx

[github-actions]

🔀 Issue Triage

This issue has been automatically assigned to @chengshuyi as the maintainer(s) of sight.

Maintainers, please review and triage this issue. Set a priority label and update the status as needed. Thanks! 🙏