fix: TOC header class, numbering config, and Lua defaults

- Add class='toc' to TOC heading in HTML export by detecting the
  configured toc_header value in render_heading()
- Add toc_header field to HtmlRenderer and wire it through
  render_page_html()
- Add toc_header, toc_header_level, html_header_numbering, and
  html_header_numbering_sym to Lua config defaults so users can
  discover and set them

crates/nuwiki-lsp/src/export.rs

@@ -259,6 +259,7 @@ pub fn render_page_html(
             cfg.html_header_numbering_sym.clone(),
         );
     }
+    r = r.with_toc_header(&cfg.toc_header);
     if !cfg.valid_html_tags.is_empty() {
         r = r.with_valid_html_tags(cfg.valid_html_tags.clone());
     }