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/providers/ollama.rs

@@ -378,10 +378,8 @@ impl OllamaProvider {
         let family = pick_first_string(map, &["family", "model_family"]);
         let mut families = pick_string_list(map, &["families", "model_families"]);
 
-        if families.is_empty()
-            && let Some(single) = family.clone()
-        {
-            families.push(single);
+        if families.is_empty() {
+            families.extend(family.clone());
         }
 
         let system = pick_first_string(map, &["system"]);