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/config.rs

@@ -1332,6 +1332,8 @@ pub struct UiSettings {
     pub show_cursor_outside_insert: bool,
     #[serde(default = "UiSettings::default_syntax_highlighting")]
     pub syntax_highlighting: bool,
+    #[serde(default = "UiSettings::default_render_markdown")]
+    pub render_markdown: bool,
     #[serde(default = "UiSettings::default_show_timestamps")]
     pub show_timestamps: bool,
     #[serde(default = "UiSettings::default_icon_mode")]
@@ -1392,6 +1394,10 @@ impl UiSettings {
         true
     }
 
+    const fn default_render_markdown() -> bool {
+        true
+    }
+
     const fn default_show_timestamps() -> bool {
         true
     }
@@ -1466,6 +1472,7 @@ impl Default for UiSettings {
             scrollback_lines: Self::default_scrollback_lines(),
             show_cursor_outside_insert: Self::default_show_cursor_outside_insert(),
             syntax_highlighting: Self::default_syntax_highlighting(),
+            render_markdown: Self::default_render_markdown(),
             show_timestamps: Self::default_show_timestamps(),
             icon_mode: Self::default_icon_mode(),
         }