Add HEIC image support on Mac

What version of the Codex App are you using (From “About Codex” dialog)?

26.325.31654 (1272)

What subscription do you have?

Plus

What platform is your computer?

Darwin 25.4.0 arm64 arm

What issue are you seeing?

On macOS, screenshots can be saved as HEIC. When dropped into Codex:
• No preview is shown (unsupported format)
• Requires manual conversion to PNG to work

Expected:
• HEIC images should preview in chat
• Or auto-convert to PNG internally

Impact:
Creates unnecessary friction for common screenshot workflows.

What steps can reproduce the bug?

1. Take a screenshot with heic format (default unless changed)
2. Drop it to Codex app
3. See the preview not supported

What is the expected behavior?

HEIC images should preview in chat

Additional information

<img width="809" height="225" alt="Image" src="https://github.com/user-attachments/assets/fd5f3733-3191-4cd1-8983-bbfe69f6cd14" />