[Feature] 会话列表样式优化

[ushaio]

Affected area

Desktop UI (agent-gui / React)

Problem statement (what should this solve)

当前全部是一种样式，视觉上无法辨别pin的会话，无法区分会话的重要等级

Proposed behavior

没有任何样式回馈，希望能对pin的会话和普通会话加个分隔栏，并且支持会话颜色标记，用于识别重要的会话内容，便于查看（交互方式，右击：添加颜色标记，显示在会话列表左侧，类似于这种引用的便利贴效果）

Estimated change scope

No response

Alternatives considered

No response

Pre-submit checklist

• I searched existing issues and pull requests and found no duplicates.
• This proposal is focused on a single feature or improvement.
• I understand a PR should come after this issue is confirmed by maintainers, otherwise it will be converted to draft.