feat: Allow pasting newlines (#866)

Noticed that when pasting multi-line blocks, each newline was treated
like a new submission.
Update tui to handle Paste directly and map newlines to shift+enter.

# Test

Copied this into clipboard:
```
Do nothing.
Explain this repo to me.
```

Pasted in and saw multi-line input. Hitting Enter then submitted the
full block.

codex-rs/tui/Cargo.toml

@@ -21,7 +21,7 @@ codex-ansi-escape = { path = "../ansi-escape" }
 codex-core = { path = "../core" }
 codex-common = { path = "../common", features = ["cli", "elapsed"] }
 color-eyre = "0.6.3"
-crossterm = "0.28.1"
+crossterm = { version = "0.28.1", features = ["bracketed-paste"] }
 mcp-types = { path = "../mcp-types" }
 ratatui = { version = "0.29.0", features = [
     "unstable-widget-ref",