Feature: Unarchive accidentally archived conversations

Problem

There is currently no way to unarchive a conversation that was accidentally archived. Once a conversation is archived, it's effectively gone from the active list with no recovery path.

This is easy to trigger accidentally — a misclick or hasty action can archive an important conversation with no undo.

Proposed Solution

Add an "unarchive" capability so users can restore accidentally archived conversations. This could take several forms:

• Undo toast: After archiving, show a brief "Undo" prompt (similar to Gmail's undo send) that lets the user immediately reverse the action
• Archive view: A /archived command or menu that lists archived conversations and allows restoring them
• Unarchive command: A /unarchive command that lists recently archived conversations for restoration

Alternatives Considered

• Making archive require confirmation — adds friction to the happy path for every archive action
• Making archive harder to trigger accidentally — still doesn't solve the problem once it happens

Additional Context

This is a quality-of-life improvement that reduces anxiety around the archive action. Users should feel confident that archiving is reversible.