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/img.html

@@ -0,0 +1,38 @@
+{{- /*
+  Renders a Hugo image resource as an optimised WebP <img> with responsive srcset.
+
+  Parameters:
+    res     — Hugo image resource (required)
+    widths  — slice of pixel widths to generate, e.g. (slice 600 1200)
+               default: (slice 800 1600)
+    sizes   — value for the <img sizes> attribute
+               default: "100vw"
+    class   — CSS classes applied to <img>
+    alt     — alt text (default "")
+    loading — "lazy" or "eager" (default "lazy")
+*/ -}}
+{{- $res     := .res -}}
+{{- $widths  := .widths  | default (slice 800 1600) -}}
+{{- $sizes   := .sizes   | default "100vw" -}}
+{{- $class   := .class   | default "" -}}
+{{- $alt     := .alt     | default "" -}}
+{{- $loading := .loading | default "lazy" -}}
+
+{{- $entries := slice -}}
+{{- $primary := $res -}}
+{{- range $i, $w := $widths -}}
+  {{- $img := $res.Resize (printf "%dx webp" $w) -}}
+  {{- $entries = $entries | append (printf "%s %dw" $img.RelPermalink $w) -}}
+  {{- if eq $i 0 -}}{{- $primary = $img -}}{{- end -}}
+{{- end -}}
+<img
+  class="{{ $class }}"
+  src="{{ $primary.RelPermalink }}"
+  srcset="{{ delimit $entries ", " }}"
+  sizes="{{ $sizes }}"
+  alt="{{ $alt }}"
+  width="{{ $primary.Width }}"
+  height="{{ $primary.Height }}"
+  loading="{{ $loading }}"
+  decoding="async"
+>