this shows `/diff` output in an overlay like the transcript, instead of dumping it into history. https://github.com/user-attachments/assets/48e79b65-7f66-45dd-97b3-d5c627ac7349
moves TranscriptApp to be an "overlay", and continue to pump AppEvents while the transcript is active, but forward all tui handling to the transcript screen.
this adds a new 'transcript mode' that shows the full event history in a "pager"-style interface. https://github.com/user-attachments/assets/52df7a14-adb2-4ea7-a0f9-7f5eb8235182