Enable macOS spellcheck (and optionally grammar) in the prompt input box

Problem: In the Codex macOS desktop app, the prompt input field does not use macOS system spellchecking, so there’s no “Check Spelling While Typing” (red underlines) or suggestion UI. This makes it easy to submit prompts with typos, and I currently have to draft prompts in another app and paste them in.

Repro:
1) In the Codex prompt box, type teh (or any misspelling).
2) Expected: macOS spellcheck underline + suggestions (as in TextEdit/Notes).
3) Actual: no underline and no suggestion UI.

Request: Enable macOS-native spellchecking for the prompt editor (e.g., NSSpellChecker / NSTextView / equivalent in the UI stack) so spellcheck-as-you-type works in the prompt box.

Why it matters: Prompts are the primary interface; typos reduce clarity and can change meaning. System spellcheck is standard on macOS input fields and improves usability immediately.

Acceptance criteria:

• Prompt input supports “Check Spelling While Typing” using the user’s macOS language settings.
• Context menu on misspelled words shows spelling suggestions.
• Works for both typed and pasted text in multi-line prompts.
• Add a Settings toggle: Spellcheck: On/Off (default On).
• (Optional) Separate toggle for Grammar checking if supported.

Environment:

• Codex: 26.212.1823 (661)
• macOS: 26.2 (25C56)

Note: GitHub Actions flagged a possible duplicate (#11846). If that’s the same request, I’m happy to close this as a duplicate and add my environment details there.