[ux] Initial prompt input area is too small for composing longer prompts

## Problem

The initial prompt input textarea in the TUI is small and constraining. When composing multi-line prompts, pasting SQL queries, or writing detailed instructions, the input area does not provide enough visible space to review the text before submitting.

## Expected behavior

- The input area should be larger by default, or auto-expand as content grows
- Ideally, support a fullscreen/expanded editor mode for composing longer prompts (the Leader + e "Open editor" keybind may address this, but it is not discoverable)
- The initial empty-state input should signal that longer prompts are welcome

## Current behavior

- The input area has a small fixed height
- Users with longer prompts must scroll within a tiny text box
- This subtly discourages detailed prompts, which hurts output quality

## Notes

- The Leader + e keybind opens an external editor — this is a good escape hatch but is not discoverable from the input area itself
- Consider adding a visual hint (e.g., "Press Ctrl+X e for full editor") near the input area