tui2: copy selection dismisses highlight (#8718)

Clicking the transcript copy pill or pressing the copy shortcut now
copies the selected transcript text and clears the highlight.

Show transient footer feedback ("Copied"/"Copy failed") after a copy
attempt, with logic in transcript_copy_action to keep app.rs smaller and
closer to tui for long-term diffs.

Update footer snapshots and add tiny unit tests for feedback expiry.


https://github.com/user-attachments/assets/c36c8163-11c5-476b-b388-e6fbe0ff6034

codex-rs/tui2/src/lib.rs

@@ -77,6 +77,7 @@ mod terminal_palette;
 mod text_formatting;
 mod tooltips;
 mod transcript_copy;
+mod transcript_copy_action;
 mod transcript_copy_ui;
 mod transcript_multi_click;
 mod transcript_render;