fix(tui): separate queued prompt dock from composer

[opencode-agent]

The queued prompt dock sits directly on top of the composer, so its gray left border runs straight into the composer's highlighted border and the two read as one broken bar.

Add a one-row bottom margin to the dock so it renders as a separate block above the composer.

Before:

  ┃
  ┃  2 queued · refactor the parser
  ┃
  ┃
  ┃
  ┃
  ┃  Build · Demo Model Demo
  ╹▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀

After:

  ┃
  ┃  2 queued · refactor the parser
  ┃

  ┃
  ┃
  ┃
  ┃  Build · Demo Model Demo
  ╹▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀

Verified with a scratch render of the session route with two queued prompts at 70 columns (scratch test not committed). bun run typecheck passes in packages/tui.

Requested by: @nexxeln (nexxel via Slack)