fix(404): noindex the error page instead of emitting a canonical to /404/

A canonical pointing at a URL that does not exist is worse than none. Normal pages
are untouched (canonical still emitted, no robots meta).

src/layouts/BaseLayout.astro

@@ -24,6 +24,9 @@ interface Props {
     tags?: string[];
     category?: string;
   };
+  // Error pages: emit noindex and skip the canonical (a canonical pointing at a
+  // page that does not exist is worse than none).
+  noindex?: boolean;
 }
 
 const {
@@ -35,6 +38,7 @@ const {
   switchHref,
   altLocaleUrl,
   article,
+  noindex = false,
 } = Astro.props;
 
 const canonical = new URL(Astro.url.pathname, SITE.url).href;
@@ -71,7 +75,7 @@ const nav = isZh
 
     <title>{title}</title>
     <meta name="description" content={description} />
-    <link rel="canonical" href={canonical} />
+    {noindex ? <meta name="robots" content="noindex, follow" /> : <link rel="canonical" href={canonical} />}
 
     <!-- hreflang: this page + its other-language twin -->
     {hreflang.map((alt) => (