The final split keeps every surface configurable: interface (labels,
controls, messages, composer chrome), prompt (textarea and placeholder
only), code (code blocks, diffs, file previews), and terminal. Every
dropdown now offers the same Default entry, fixing the prompt/terminal
inconsistency.

The custom input no longer commits on every keystroke: it edits a local
draft and commits 400ms after typing pauses (or on Enter/blur), so the app
does not reflow mid-word and the unknown-name flag waits for the pause.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>