perf: Hugo image processing — auto-convert all images to WebP with srcset

Add layouts/partials/img.html helper that resizes and converts any Hugo
page-resource image to WebP, emitting a responsive srcset at multiple
widths. Wire it up to every image rendering site:

- post-card.html: thumbnail (600w/1000w) + avatar (64px WebP)
- post-card-large.html: featured card background (800w/1200w)
- posts/single.html: banner (1200w/1800w, eager) + gallery (800w/1200w)
- author-card.html: avatar (96px WebP, 2× retina)

Result: banner.png 7.9 MB → 496 KB WebP at 1800w (−94 %).
Hugo caches processed images in resources/_gen/ across builds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

layouts/partials/author-card.html

@@ -3,18 +3,16 @@
 */ -}}
 <div class="flex items-center gap-4 py-6 border-t border-b border-zinc my-8">
   {{- $avatarRes := .Resources.GetMatch "avatar.*" -}}
-  {{- $avatarSrc := "" -}}
-  {{- with $avatarRes }}{{ $avatarSrc = .RelPermalink }}{{ else }}{{ with $.Params.avatar }}{{ $avatarSrc = . }}{{ end }}{{ end -}}
-  {{- with $avatarSrc -}}
-  <a href="{{ $.RelPermalink }}" class="flex-shrink-0">
-    <img
-      src="{{ . }}"
-      alt="{{ $.Params.name | default $.Title }}"
-      class="w-12 h-12 object-cover border border-zinc hover:border-heat transition-colors"
-      loading="lazy"
-    >
-  </a>
-  {{- end -}}
+  {{- if $avatarRes -}}
+    {{- $av := $avatarRes.Resize "96x webp" -}}
+    <a href="{{ $.RelPermalink }}" class="flex-shrink-0">
+      <img src="{{ $av.RelPermalink }}" alt="{{ $.Params.name | default $.Title }}" class="w-12 h-12 object-cover border border-zinc hover:border-heat transition-colors" width="{{ $av.Width }}" height="{{ $av.Height }}" loading="lazy" decoding="async">
+    </a>
+  {{- else -}}{{- with .Params.avatar -}}
+    <a href="{{ $.RelPermalink }}" class="flex-shrink-0">
+      <img src="{{ . }}" alt="{{ $.Params.name | default $.Title }}" class="w-12 h-12 object-cover border border-zinc hover:border-heat transition-colors" loading="lazy" decoding="async">
+    </a>
+  {{- end -}}{{- end -}}
   <div class="min-w-0">
     <a href="{{ .RelPermalink }}" class="label text-heat hover:text-frost transition-colors block mb-1">
       {{ .Params.name | default .Title }}