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/config.rs

@@ -176,6 +176,10 @@ pub struct HtmlConfig {
     /// vimwiki `html_header_numbering_sym`: symbol appended after the
     /// section number (e.g. `.` → `1.2. Heading`). Empty by default.
     pub html_header_numbering_sym: String,
+    /// vimwiki `toc_header`: the heading text that identifies the TOC
+    /// section. When a heading matches this value (case-insensitive), it
+    /// gets `class="toc"` in HTML export.
+    pub toc_header: String,
     /// vimwiki `rss_name`: filename of the generated RSS feed (default
     /// `rss.xml`), relative to `html_path`.
     pub rss_name: String,
@@ -224,6 +228,7 @@ impl Default for HtmlConfig {
             base_url: String::new(),
             html_header_numbering: 0,
             html_header_numbering_sym: String::new(),
+            toc_header: default_toc_header(),
             rss_name: "rss.xml".into(),
             rss_max_items: 10,
             valid_html_tags: default_valid_html_tags(),
@@ -954,6 +959,9 @@ impl From<RawWiki> for WikiConfig {
         if let Some(s) = r.html_header_numbering_sym {
             html.html_header_numbering_sym = s;
         }
+        if let Some(ref s) = r.toc_header {
+            html.toc_header = s.clone();
+        }
         if let Some(s) = r.rss_name {
             html.rss_name = s;
         }