Add optional horizontal separator lines around the TUI chat composer

What problem does this solve?

The Codex CLI TUI currently supports syntax/theme customization, but the bottom chat composer has little visual separation from the transcript. On light themes especially, the input area can feel visually flat and harder to distinguish.

I would like an optional setting to render only a top and bottom separator line around the user input composer, without drawing a full box border.

Proposed behavior

Add a small TUI config option, for example:

[tui.composer]
horizontal_lines = true
line_color = "dark-gray"

When enabled, the composer could render like:

────────────────────────────────────────
› Ask Codex to do anything
────────────────────────────────────────

This is intentionally lighter than a full border. It only adds the two horizontal separator lines above and below the input area.

Why not just use a theme?

Existing /theme support changes syntax highlighting and related colors, but it does not expose a way to visually separate the composer region itself.

Implementation idea

The likely rendering point is codex-rs/tui/src/bottom_pane/chat_composer.rs, where the composer background block is currently rendered. This could potentially use Borders::TOP | Borders::BOTTOM on the existing composer Block, gated behind config.

Notes

This would be useful for users who prefer a stronger visual boundary around the active input area, while keeping the default UI unchanged.