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/Cargo.toml

@@ -40,23 +40,20 @@ codex-login = { workspace = true }
 codex-ollama = { workspace = true }
 codex-protocol = { workspace = true }
 color-eyre = { workspace = true }
-crossterm = { workspace = true, features = [
-    "bracketed-paste",
-    "event-stream",
-] }
-dirs = { workspace = true }
+crossterm = { workspace = true, features = ["bracketed-paste", "event-stream"] }
 diffy = { workspace = true }
-image = { workspace = true, features = [
-    "jpeg",
-    "png",
-] }
+dirs = { workspace = true }
+image = { workspace = true, features = ["jpeg", "png"] }
 itertools = { workspace = true }
 lazy_static = { workspace = true }
 mcp-types = { workspace = true }
 path-clean = { workspace = true }
+pathdiff = { workspace = true }
+pulldown-cmark = { workspace = true }
 rand = { workspace = true }
 ratatui = { workspace = true, features = [
     "scrolling-regions",
+    "unstable-backend-writer",
     "unstable-rendered-line-info",
     "unstable-widget-ref",
 ] }
@@ -80,11 +77,9 @@ tokio-stream = { workspace = true }
 tracing = { workspace = true, features = ["log"] }
 tracing-appender = { workspace = true }
 tracing-subscriber = { workspace = true, features = ["env-filter"] }
-pulldown-cmark = { workspace = true }
 unicode-segmentation = { workspace = true }
 unicode-width = { workspace = true }
 url = { workspace = true }
-pathdiff = { workspace = true }
 
 [target.'cfg(unix)'.dependencies]
 libc = { workspace = true }