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-large.html

@@ -10,7 +10,7 @@
     {{- if not $thumb -}}{{- $thumb = $post.Resources.GetMatch "01.png" -}}{{- end -}}
     <div class="card-media w-full h-full absolute inset-0">
       {{- 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 800 1200) "sizes" "(max-width: 768px) 100vw, 50vw" "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 -}}