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.

Cargo.toml

@@ -9,6 +9,7 @@ members = [
     "crates/owlen-mcp-client",
     "crates/owlen-mcp-code-server",
     "crates/owlen-mcp-prompt-server",
+    "crates/owlen-markdown",
 ]
 exclude = []