Use a branded landscape social card instead of the raw portrait cover

The book cover is a 1664x2432 portrait image, which crops badly into
og:image's expected 1200x630. Add a social_image field (a composited
card: cover centered on the site's parchment background) that og:image/
twitter:card prefer, falling back to cover_image if unset.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

layouts/_default/baseof.html

@@ -5,7 +5,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <title>{{ if .IsHome }}{{ .Site.Title }}{{ else }}{{ .Title }} · {{ .Site.Title }}{{ end }}</title>
   <meta name="description" content="{{ .Description | default .Site.Params.description }}">
-  {{ with .Params.cover_image }}
+  {{ with .Params.social_image | default .Params.cover_image }}
   <meta property="og:title" content="{{ $.Title }}">
   <meta property="og:description" content="{{ $.Description | default $.Site.Params.description }}">
   <meta property="og:image" content="{{ . }}">