Acceptance Criteria:
- Workspace builds against ratatui 0.29, crossterm 0.28.1, and tui-textarea 0.7 with palette support enabled
- Chat header context and usage gauges render with refreshed tailwind gradients
- Header layout uses the Flex API to balance top-row metadata across window widths
Test Notes:
- cargo test -p owlen-tui
- Introduce new `owlen-markdown` crate that converts Markdown strings to `ratatui::Text` with headings, lists, bold/italic, and inline code.
- Add `render_markdown` config option (default true) and expose it via `app.render_markdown_enabled()`.
- Implement `:markdown [on|off]` command to toggle markdown rendering.
- Update help overlay to document the new markdown toggle.
- Adjust UI rendering to conditionally apply markdown styling based on the markdown flag and code mode.
- Wire the new crate into `owlen-tui` Cargo.toml.