- Introduce `RoleLabelDisplay` enum (inline, above, none) and integrate it into UI rendering and message formatting.
- Replace `show_role_labels` boolean with `role_label_mode` across config, formatter, session, and TUI components.
- Add `syntax_highlighting` boolean to UI settings with default `false` and support in message rendering.
- Update configuration schema version to 1.3.0 and provide deserialization handling for legacy boolean values.
- Extend theme definitions with code block styling fields (background, border, text, keyword, string, comment) and default values in `Theme`.
- Adjust related modules (`formatting.rs`, `ui.rs`, `session.rs`, `chat_app.rs`) to use the new settings and theme fields.
- Added a `tool_output` color to the `Theme` struct.
- Updated all built-in themes to include the new color.
- Modified the TUI to use the `tool_output` color for rendering tool output.
- Introduce multiple built-in themes (`default_dark`, `default_light`, `gruvbox`, `dracula`, `solarized`, `midnight-ocean`, `rose-pine`, `monokai`, `material-dark`, `material-light`).
- Implement theming system with customizable color schemes for all UI components in the TUI.
- Include documentation for themes in `themes/README.md`.
- Add fallback mechanisms for default themes in case of parsing errors.
- Support custom themes with overrides via configuration.
