Add unstash/pop keybinding to recover stashed messages

Problem

Pressing Ctrl+S stashes the current input message, but there is no corresponding unstash/pop action to retrieve it. If you accidentally press Ctrl+S, your in-progress message is lost with no way to recover it.

Proposed Solution

Add a chat:unstash (or chat:pop) action that restores the most recently stashed message back into the input field, and assign it a default keybinding (e.g. Ctrl+Shift+S).