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/src/commands/mod.rs

@@ -148,6 +148,10 @@ const COMMANDS: &[CommandSpec] = &[
         keyword: "reload",
         description: "Reload configuration and themes",
     },
+    CommandSpec {
+        keyword: "markdown",
+        description: "Toggle markdown rendering",
+    },
     CommandSpec {
         keyword: "e",
         description: "Edit a file",