Fix multiline markdown paste corruption in the TUI

[lespaceman]

What to build

Fix the TUI text-entry/rendering path so pasting multiline markdown-heavy content into Athena Flow does not corrupt the visible screen. The pasted content should remain intact, and surrounding UI regions such as the header, feed, action table, and input area should continue to wrap, scroll, and repaint predictably.

This issue was raised from a screenshot where pasting a markdown report into the TUI caused overlapping columns and broken layout while running the exploratory-testing workflow.

Acceptance criteria

• Pasting multiline markdown into the prompt/input area preserves the pasted text without dropping or reordering lines.
• Markdown structures such as bullets, headings, code fences, and long lines wrap or scroll predictably in the TUI.
• The header, feed, details/action table, and input area do not visually overlap after the paste.
• A regression test covers the multiline paste or equivalent text insertion/render path.

Blocked by

None - can start immediately