feat(prompt): undo queued prompts back into the input - #51124

Summary

• Add Undo to queued prompts so a queued prompt can be pulled back into the input for editing.
• Full TUI and Mini: ctrl+u (queued_prompt.undo) in the queued prompts picker, listed next to delete.
• App: icon-only Undo button on queued rows (tooltip + accessible name), between Steer and Remove.
• If the input already has text, the prompt is appended after a blank line instead of replacing the draft, so several prompts can be undone in a row. File/agent/skill mentions, pasted blocks and image attachments keep their positions.
• Undo is refused only while in shell mode with text typed. The app also keeps prompts with hidden file context in the queue rather than dropping that context.
• Input is read-only while the cancel is pending so typing can't be overwritten.

Requested by: @vimtor (victor via Slack)