fix(export): emit one CSS class per keyword for independent colouring

All keywords previously shared `class="todo"`, so a stylesheet couldn't
distinguish e.g. red TODO from green DONE. Emit a distinct class per keyword
(todo/done/started/fixme/fixed/xxx); TODO keeps the vimwiki `todo` class so
stock vimwiki CSS still styles it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-core/src/render/html.rs

@@ -492,13 +492,16 @@ impl HtmlRenderer {
     }
 
     fn render_keyword(&self, n: &KeywordNode, w: &mut dyn Write) -> io::Result<()> {
+        // One class per keyword so stylesheets can colour them independently
+        // (e.g. red TODO/FIXME/XXX vs green DONE/FIXED/STARTED). TODO keeps the
+        // vimwiki `todo` class so stock vimwiki CSS still styles it.
         let (label, class) = match n.keyword {
-            Keyword::Todo => ("TODO", "keyword keyword-todo"),
-            Keyword::Done => ("DONE", "keyword keyword-done"),
-            Keyword::Started => ("STARTED", "keyword keyword-started"),
-            Keyword::Fixme => ("FIXME", "keyword keyword-fixme"),
-            Keyword::Fixed => ("FIXED", "keyword keyword-fixed"),
-            Keyword::Xxx => ("XXX", "keyword keyword-xxx"),
+            Keyword::Todo => ("TODO", "todo"),
+            Keyword::Done => ("DONE", "done"),
+            Keyword::Started => ("STARTED", "started"),
+            Keyword::Fixme => ("FIXME", "fixme"),
+            Keyword::Fixed => ("FIXED", "fixed"),
+            Keyword::Xxx => ("XXX", "xxx"),
         };
         write!(w, "<span class=\"{class}\">{label}</span>")
     }