feat: support leaf bundles for blog posts, refactor header images

themes/hugo-sustain/layouts/partials/blog/headerimage.html

@@ -0,0 +1,15 @@
+{{- $page := . -}}
+{{- with $page.Params.headerimage }}
+  <div class="header-image">
+    {{- if eq $page.BundleType "leaf" }}
+      {{- with $page.Resources.GetMatch .src }}
+        <img src="{{ .RelPermalink }}" />
+      {{- end }}
+    {{- else }}
+      {{- with resources.GetMatch (printf "**/%s" .src) }}
+        <img src="{{ .RelPermalink }}" />
+      {{- end }}
+    {{- end }}
+    {{- with .text }}<p>{{ . | markdownify }}</p>{{ end }}
+  </div>
+{{- end }}