feat(app): restore queued follow-up mode

[Mr-V1be]

Summary

• restore the Queue/Steer follow-up setting in the app UI and settings store
• implement client-side tail queuing so queued follow-ups wait for session.idle before calling promptAsync
• show a queued badge for locally delayed follow-up messages so they stay visible in the timeline

Why

The previous Queue option was removed because follow-up prompts were being dequeued at the wrong time. This keeps the existing Steer behavior as the default, but restores a real delayed queue mode for users who want follow-ups to start only after the current turn fully finishes.

Testing

• bun test --preload ./happydom.ts ./src/components/prompt-input/submit.test.ts (run from packages/app)
• bun run typecheck (run from packages/app)
• bun run build (run from packages/app)

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found 2 potentially related PRs (excluding the current PR #21779):

1. PR feat: queue follow-ups before dispatch #21491 - feat: queue follow-ups before dispatch

   • feat: queue follow-ups before dispatch #21491
   • Likely related as it deals with queuing follow-ups

2. PR fix(app): avoid false follow-up queue after session becomes idle #18944 - fix(app): avoid false follow-up queue after session becomes idle

   • fix(app): avoid false follow-up queue after session becomes idle #18944
   • Related to follow-up queueing behavior and session.idle timing

3. PR feat(app): add queued follow-up queue/steer controls for web UI #13923 - feat(app): add queued follow-up queue/steer controls for web UI

   • feat(app): add queued follow-up queue/steer controls for web UI #13923
   • Directly related to adding Queue/Steer controls for follow-ups in the UI

These PRs address similar aspects of follow-up queuing, Queue/Steer controls, and session idle behavior. PR #21491 in particular appears to be actively addressing follow-up queuing, which is core to this PR's implementation. You may want to check if these are addressing the same issue or if they're complementary changes.

[github-actions]

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