Task 18: Chat UI Textarea Overlay and Border Styling Fix

id: 18 title: Chat UI Textarea Overlay and Border Styling Fix status: Not started summary: Fix overlay of waiting messages and streamline borders between chat window and input area to improve visibility and reclaim terminal space. goal: | Adjust the TUI chat interface so that waiting/status messages no longer overlay the first line of the input textarea (ensuring user drafts remain visible), and merge/remove borders as follows: - Merge the bottom border of the chat history window with the top border of the input textarea. - Remove the left, right, and bottom overall borders around the chat interface to reduce wasted space.

This task is specific to codex-rs.

Waiting/status messages (e.g. "Thinking...", "Typing...", etc.) appear above the textarea rather than overlaying the first line of the input area.
User draft text remains visible at all times, even when agent messages or status indicators are rendered.
The bottom border of the chat history pane and the top border of the textarea are unified into a single border line.
The left, right, and bottom borders around the entire chat UI are removed, reclaiming columns/rows in the terminal.
Manual or automated visual verification steps demonstrate correct layout in a variety of terminal widths.

How it was implemented
(Not implemented yet)

How it works
(Not implemented yet)

This involves updating the rendering logic in the TUI modules (likely under tui/src/ in codex-rs).
Ensure layout changes do not break existing tests or rendering in unusual terminal sizes.
Consider writing a simple snapshot test or manual demo script to validate border and overlay behavior.