[sight] feat: 优化代码对比视图的可读性

[husterL9]

Component

sight

Problem Statement

Token Savings 面板中展示的优化前后代码对比缺乏直观的解释，普通用户难以理解具体的节省逻辑。原有 DiffView 组件仅以整块红/绿背景展示原始与优化后文本，没有行级对比，没有自然语言说明，diff_lines 字段始终为空。

Proposed Solution

1. 后端新增 optimization_reason 字段，根据操作类型+token变化量自动生成中文自然语言说明
2. 后端实现 compute_diff_lines 函数，基于 LCS 算法生成行级 diff 数据
3. 前端 DiffView 重构：顶部蓝色优化说明面板 + 行级 diff 高亮 + 底部统计
4. 前端表格行增强：显示优化标题和节省百分比

Alternatives Considered

• 仅前端实现 diff 算法（缺点：大文本时前端性能不佳）
• 引入第三方 diff 库（缺点：增加依赖体积）

Additional Context

对应需求 ID: 82706233

[github-actions]

🔀 Issue Triage

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

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