Feature request: bulk archive sessions in Ungrouped

[Erick-Abe-git]

Problem

The Web sidebar supports archiving a session only one row at a time. When Ungrouped contains many old sessions, clearing the list requires repeating hover → session actions → Archive session for every row.

Proposal

Add bulk archive controls for the Ungrouped section:

• multi-select sessions, then archive the selected sessions;
• an Archive all action for the section;
• show the affected session count and offer an immediate Undo action.

The operation should keep the current archive semantics: hide sessions from the sidebar without deleting their persisted conversation data.

中文

问题

当前 Web 侧边栏只能逐条归档会话。若“未分组”中积累了许多旧会话，需要对每一项重复执行“悬停 → 会话操作 → 归档会话”。

建议

为“未分组”提供批量归档：

• 多选后归档所选会话；
• 提供“全部归档”操作；
• 显示影响的会话数量，并提供即时撤销。

归档应保持现有语义：仅从侧边栏隐藏，不删除已持久化的会话记录。