Composer text becomes unreadable after live terminal theme switch

## Repro

1. Start Codex TUI in a terminal with a dark theme.
2. Type in the composer.
3. Switch the terminal to a light theme without restarting Codex.
4. Return focus to Codex and keep typing.

## Actual
Composer input becomes nearly invisible, effectively white/light text on a white/light background.

## Expected
Composer text should stay readable after a terminal theme change.

## Suspected cause
The composer background is recomputed from terminal colors, but textarea text is rendered with default/inherited cell styling. After a dark -> light theme switch, stale
foreground styling can remain while the background updates, leaving the input white-on-white.

## Relevant files

• codex-rs/tui/src/bottom_pane/chat_composer.rs
• codex-rs/tui/src/bottom_pane/textarea.rs
• codex-rs/tui/src/tui/event_stream.rs

Before switch:

<img width="2557" height="702" alt="Image" src="https://github.com/user-attachments/assets/4cdae374-f963-41c5-8b6c-8dac98093eb1" />
After:

<img width="2557" height="702" alt="Image" src="https://github.com/user-attachments/assets/39a368a8-48b0-49f5-bce7-ecd55b05a1c7" />

Same after dark to light switches

# Evidence:

• Composer background uses user_message_style():

<https://github.com/openai/codex/blob/main/codex-rs/tui/src/bottom_pane/chat_composer.rs#L4380>
<https://github.com/openai/codex/blob/main/codex-rs/tui/src/style.rs#L8>

• Typed text uses plain Style::default():

<https://github.com/openai/codex/blob/main/codex-rs/tui/src/bottom_pane/textarea.rs#L1373>

• Palette refresh only happens on FocusGained:

<https://github.com/openai/codex/blob/main/codex-rs/tui/src/tui/event_stream.rs#L249>

• Style guide warning about white/black assumptions:

<https://github.com/openai/codex/blob/main/codex-rs/tui/styles.md#L17>