Why: a busy conversation rejected further input, and the file list lived
only behind the workspace overlay.
Evidence: npx tsc --noEmit and vitest 42 tests, including outbox
append/edit/promote. The queue uses Collapsible and design tokens.
The files tab reuses ArtifactsPanel.
Impact: while a run is active, Enter queues the draft (edit, delete, or
send-now). Finished and failed runs drain the queue. HTTP 409 also
enqueues instead of only toasting. Backend serialization is unchanged.
Refs: task feature/input-queue
Co-authored-by: Cursor <cursoragent@cursor.com>