update composer + user message styling (#4240)

Changes:

- the composer and user messages now have a colored background that
stretches the entire width of the terminal.
- the prompt character was changed from a cyan `▌` to a bold `›`.
- the "working" shimmer now follows the "dark gray" color of the
terminal, better matching the terminal's color scheme

| Terminal + Background        | Screenshot |
|------------------------------|------------|
| iTerm with dark bg | <img width="810" height="641" alt="Screenshot
2025-09-25 at 11 44 52 AM"
src="https://github.com/user-attachments/assets/1317e579-64a9-4785-93e6-98b0258f5d92"
/> |
| iTerm with light bg | <img width="845" height="540" alt="Screenshot
2025-09-25 at 11 46 29 AM"
src="https://github.com/user-attachments/assets/e671d490-c747-4460-af0b-3f8d7f7a6b8e"
/> |
| iTerm with color bg | <img width="825" height="564" alt="Screenshot
2025-09-25 at 11 47 12 AM"
src="https://github.com/user-attachments/assets/141cda1b-1164-41d5-87da-3be11e6a3063"
/> |
| Terminal.app with dark bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 22 AM"
src="https://github.com/user-attachments/assets/93fc4781-99f7-4ee7-9c8e-3db3cd854fe5"
/> |
| Terminal.app with light bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 46 04 AM"
src="https://github.com/user-attachments/assets/19bf6a3c-91e0-447b-9667-b8033f512219"
/> |
| Terminal.app with color bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 50 AM"
src="https://github.com/user-attachments/assets/dd7c4b5b-342e-4028-8140-f4e65752bd0b"
/> |

codex-rs/tui/src/tui.rs

@@ -1,3 +1,4 @@
+use std::io::IsTerminal;
 use std::io::Result;
 use std::io::Stdout;
 use std::io::stdout;
@@ -123,6 +124,9 @@ pub fn restore() -> Result<()> {
 
 /// Initialize the terminal (inline viewport; history stays in normal scrollback)
 pub fn init() -> Result<Terminal> {
+    if !stdout().is_terminal() {
+        return Err(std::io::Error::other("stdout is not a terminal"));
+    }
     set_modes()?;
 
     set_panic_hook();
@@ -271,6 +275,10 @@ impl Tui {
         // Detect keyboard enhancement support before any EventStream is created so the
         // crossterm poller can acquire its lock without contention.
         let enhanced_keys_supported = supports_keyboard_enhancement().unwrap_or(false);
+        // Cache this to avoid contention with the event reader.
+        supports_color::on_cached(supports_color::Stream::Stdout);
+        let _ = crate::terminal_palette::terminal_palette();
+        let _ = crate::terminal_palette::default_colors();
 
         Self {
             frame_schedule_tx,