Finish the Neon Tan reader-magnet page

Fill in the gift page's copy (title, description, subhead), wire the
cover image into the hero and og:image/Twitter card meta, and point the
download links at the real bucket path. Also fixes a bug where every
page's meta description fell back to the site-wide bio instead of its
own.

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

layouts/_default/baseof.html

@@ -4,7 +4,13 @@
   <meta charset="utf-8">
   <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="{{ .Site.Params.description }}">
+  <meta name="description" content="{{ .Description | default .Site.Params.description }}">
+  {{ with .Params.cover_image }}
+  <meta property="og:title" content="{{ $.Title }}">
+  <meta property="og:description" content="{{ $.Description | default $.Site.Params.description }}">
+  <meta property="og:image" content="{{ . }}">
+  <meta name="twitter:card" content="summary_large_image">
+  {{ end }}
   {{ $css := resources.Get "css/main.css" }}
   {{ if hugo.IsProduction }}
     {{ $css = $css | minify | fingerprint }}