Feature request: configurable colors to distinguish user messages from assistant replies

Problem

In the terminal, user messages and assistant replies are rendered with very similar (or default) styling, making it hard to visually distinguish who said what—especially during long sessions.

Suggestion

Add configuration options (e.g. in config.toml) to customize the foreground/background colors for:

• User messages (echoed prompt)
• Assistant messages (streamed response / final output)

For example:

[ui.colors]
user_message = "bold cyan"
assistant_message = "default"

Alternatives

• Automatically assign different default styles under the existing dark/light themes.
• Expose a message_style enum like compact, conversation, bare.

Additional context

Currently theme = "dark" only affects diff colors, task browser, toolbars, etc., but not the main conversation text. This would greatly improve readability for users who spend a lot of time in the CLI.

Start by locating config.toml and the existing dark/light theme color handling, then inspect where conversation text is rendered in the terminal. Done means user and assistant messages support independent foreground/background styles through configuration, with the requested example behavior working.