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

lua/nuwiki/config.lua

@@ -35,10 +35,19 @@ M.defaults = {
   --   custom_wiki2html, custom_wiki2html_args, base_url,
   --   toc_header, toc_header_level, links_header, links_header_level,
   --   tags_header, tags_header_level,
+  --   html_header_numbering, html_header_numbering_sym,
   --   listsyms, listsym_rejected, listsyms_propagate, list_margin,
   --   links_space_char
   wikis = nil,
 
+  -- TOC defaults (per-wiki, also accepted as top-level for single-wiki setups)
+  toc_header            = 'Contents',
+  toc_header_level      = 1,
+
+  -- HTML export defaults (per-wiki, also accepted as top-level for single-wiki setups)
+  html_header_numbering      = 0,
+  html_header_numbering_sym  = '',
+
   -- Per-buffer glue. Each subgroup mirrors vimwiki's
   -- `g:vimwiki_key_mappings` shape and can be flipped off
   -- independently to suppress that group of keymaps.