Problem

The backend fully supports session archiving (Session.setArchived, PATCH route, SDK session.update), but the TUI has no way to archive or unarchive sessions. This was noted in #12020 where desktop archive was temporarily disabled because there was no way to view or restore archived sessions.

Proposed Solution

Add archive/unarchive support to the TUI:

• /archive slash command to archive the current session
• Archive/unarchive keybind actions in the session list dialog
• tab toggle in the session list to switch between active and archived sessions
• Auto-navigate home when the current session is archived
• Selecting an archived session unarchives it