Failed queue cancellation hides work that is still scheduled on the host #923

Priority: P1. Reproduced/reviewed at commit 6c7a6fbe on 2026-09-05.

Cancel removes a queued follow-up from the UI before the host confirms cancellation. If the call fails, the queue stays hidden while the host still holds work that can run at turn end.

Reproduction

1. Open a working thread with a persisted queued follow-up.
2. Make api.threads.setQueued({prompt:null}) reject.
3. Click the queued follow-up's Cancel action.
4. The queue strip disappears.
5. Read the thread from the host/fakeCoder: its queued prompt is unchanged.

Confirmed in the mounted App with the host queue and rendered queue checked separately.

Cause

useCoder.cancelQueued optimistically replaces queued with null. The catch only sets the global run error and never restores or reloads the queue.

Acceptance

• Keep the queue visible until cancellation is confirmed, or reconcile it on failure.
• Clearly indicate a failed cancellation and offer a retry.
• Do not overwrite a newer queue update when reconciling.
• Verify that UI and host agree after both success and failure; a failed cancellation must not look successful.

Source: src/useCoder.ts#L830.

Requested disposition: separate Planboard issue, plan:todo. Scope is the concrete defect above.