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/post-card.html

@@ -10,7 +10,7 @@
     {{- if not $thumb -}}{{- $thumb = $post.Resources.GetMatch "01.png" -}}{{- end -}}
     <div class="card-media aspect-editorial">
       {{- if $thumb -}}
-        <img class="w-full h-full object-cover" src="{{ $thumb.RelPermalink }}" alt="{{ $post.Title }}" loading="lazy" decoding="async">
+        {{- partial "img.html" (dict "res" $thumb "widths" (slice 600 1000) "sizes" "(max-width: 640px) 100vw, (max-width: 1024px) 50vw, 25vw" "class" "w-full h-full object-cover" "alt" $post.Title) -}}
       {{- else -}}{{- with $post.Params.banner -}}
         {{- partial "media.html" (dict "media" . "class" "w-full h-full object-cover") -}}
       {{- else -}}
@@ -40,16 +40,12 @@
         {{- $authorPage := site.GetPage (printf "authors/%s" .) -}}
         {{- with $authorPage -}}
           {{- $avRes := .Resources.GetMatch "avatar.*" -}}
-          {{- $avSrc := "" -}}
-          {{- with $avRes }}{{ $avSrc = .RelPermalink }}{{ else }}{{ with $.Params.avatar }}{{ $avSrc = . }}{{ end }}{{ end -}}
-          {{- with $avSrc -}}
-          <img
-            src="{{ . }}"
-            alt="{{ $authorPage.Params.name | default $authorPage.Title }}"
-            class="w-5 h-5 object-cover border border-smoke flex-shrink-0"
-            loading="lazy"
-          >
-          {{- end -}}
+          {{- if $avRes -}}
+            {{- $av := $avRes.Resize "64x webp" -}}
+            <img src="{{ $av.RelPermalink }}" alt="{{ $authorPage.Params.name | default $authorPage.Title }}" class="w-5 h-5 object-cover border border-smoke flex-shrink-0" width="{{ $av.Width }}" height="{{ $av.Height }}" loading="lazy" decoding="async">
+          {{- else -}}{{- with .Params.avatar -}}
+            <img src="{{ . }}" alt="{{ $authorPage.Params.name | default $authorPage.Title }}" class="w-5 h-5 object-cover border border-smoke flex-shrink-0" loading="lazy" decoding="async">
+          {{- end -}}{{- end -}}
           <span class="label text-fog">{{ .Params.name | default .Title }}</span>
           <span class="text-smoke" aria-hidden="true">/</span>
         {{- end -}}