[Mobile] Add “Edit from here” / checkpoint revert for user messages

[NANAMINER]

The web/desktop client already has an “Edit from here” action on eligible user messages, which lets you roll the thread back to that point and continue from there.

The mobile app currently has no equivalent action.

This is a significant workflow gap on mobile. A very common coding-agent workflow is:

1. Send a prompt.
2. Realize that the prompt was wrong, incomplete, or sent too early.
3. Stop the agent if necessary.
4. Go back to that user message.
5. Revert the conversation/checkpoint to that point.
6. Edit the prompt and send the corrected version.

On Android, steps 4–6 currently cannot be done from the T3 app. There is no “Edit from here” / revert action on user messages, so the only workaround is to switch to the web/desktop client.

Expected behavior:

• Add an “Edit from here” action to user messages on mobile, ideally in the long-press/message actions menu.
• Use the same checkpoint/conversation rollback behavior as the existing web implementation.
• Restore the selected user message into the composer so it can be edited and resent.
• Preserve attachments and composer context where possible.
• Make it available for any user message that is eligible for revert, not only the most recent message.

This should ideally be feature-parity with the existing web/desktop “Edit from here” button rather than a separate mobile-specific behavior.

Related:

• [Bug]: Cannot stop a running turn, edit the last prompt, and restart #11083 — Cannot stop a running turn, edit the last prompt, and restart
• [Feature]: Esc to pop a just-sent message back into the composer #6219 — Esc to pop a just-sent message back into the composer

The key difference from #6219 is that this request is not only about immediately undoing the most recently sent prompt. It is about exposing the existing checkpoint revert / “Edit from here” workflow in the mobile UI.

Setup:

• Android app
• Remote T3 server
• OpenCode provider
• Server v0.0.42