feat(tui): add markdown rendering support and toggle command

- 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.

crates/owlen-core/src/lib.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::collapsible_if)] // TODO: Remove once we can rely on Rust 2024 let-chains
+
 //! Core traits and types for OWLEN LLM client
 //!
 //! This crate provides the foundational abstractions for building