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-tui/Cargo.toml

@@ -29,6 +29,7 @@ dirs = { workspace = true }
 toml = { workspace = true }
 syntect = "5.3"
 once_cell = "1.19"
+owlen-markdown = { path = "../owlen-markdown" }
 
 # Async runtime
 tokio = { workspace = true }