feat(html): html_header_numbering section numbering for HTML export

Mirror vimwiki's html_header_numbering / html_header_numbering_sym:
top-level headings at or below the start level get a dotted section
number (1, 1.1, 1.2, ...) with a configurable trailing symbol. Off by
default (level 0). Nested headings in lists/quotes stay unnumbered,
matching upstream's document-level-only scan.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/src/export.rs

@@ -252,6 +252,12 @@ pub fn render_page_html(
     if !cfg.color_dic.is_empty() {
         r = r.with_colors(cfg.color_dic.clone());
     }
+    if cfg.html_header_numbering > 0 {
+        r = r.with_header_numbering(
+            cfg.html_header_numbering,
+            cfg.html_header_numbering_sym.clone(),
+        );
+    }
     r.render_to_string(doc)
 }