feat: 建议增加“已归档”文件的查看按钮。

### 使用场景 / Use case

已归档的会话，想回顾但是找不到查看入口。

### 当前问题 / Current limitation

目前没有查看已归档文件的地方。建议参考有个叫lody的软件，在主界面上就有查看“已归档”文件的按钮（https://lody.ai/zh/home）

### 期望方案 / Proposed solution

### 已考虑的替代方案 / Alternatives considered

_No response_

No file or test is named. Start by tracing the main interface and the existing archived-conversation flow, using the proposed screenshot as the UI reference. Done means users can find a visible entry point and open a list or view of archived conversations.

Written by the indexing model from the issue text.