[Feature]: Add an obvious archive action to the mobile conversation list

[neocody]

Problem or use case

The T3 Code mobile app does not expose a discoverable way to archive a conversation from the conversation list or from inside the conversation. Without an archive control, completed conversations remain mixed into the active list and become harder to manage over time.

I am reporting this as a feature request because the mobile control appears to be absent, although it also feels like a desktop/mobile parity gap.

Proposed behavior

• Add an Archive action to the conversation overflow or long-press menu.
• Optionally support swipe-to-archive from the mobile conversation list.
• Provide a discoverable Archived view and an Unarchive action.
• Do not permanently delete conversation history when archiving.

Prior work

This appears related to #5204 and the older closed implementation attempt #2045. Both are closed, but the capability is still not available or discoverable in the current mobile app.

[neocody]

Closing this as a duplicate of #7026, the earlier open discussion covering the same mobile archive and recovery flow. I added the independent iOS discoverability feedback there so it is consolidated in the canonical request.