fix(threads): recover cleanly when turn/steer fails

[amanthanvi]

Summary

Fix confusing stuck states when a follow-up message is sent as turn/steer and steering fails.

Part of #343.

Problem

Current steer failure paths can leave processing/active-turn state stale, and the user can end up in unclear queue/steer behavior with no obvious recovery path.

Scope

• Extend send flow with explicit send intents (default | queue | steer)
• Handle steer error responses and exceptions distinctly
• On steer failure:
  1. clear stale processing + active-turn state
  2. automatically enqueue the message payload (text/images/app mentions)
  3. show clear user-facing message indicating steer failed and message was queued
• Keep queue pause/flush behavior for request-user-input states unchanged

Acceptance Criteria

• Steer RPC error clears stale state and queues the message
• Steer exception path behaves the same
• Queued fallback messages remain editable/deletable and flush normally
• No fallback to turn/start from a failed steer attempt
• Tests cover steer-failure recovery and queue fallback behavior