Fix Hugo v0.156+ deprecations: Site.Author, Site.Data, and libsass

- Replace .Site.Author.{email,name} with .Site.Params.{social.email,author} in rss.xml
- Replace .Site.Data with hugo.Data in footer, social, and contact-box templates
- Switch SCSS transpiler from libsass to dartsass; add @use "sass:color" and replace darken()/lighten() with color.adjust()

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

themes/hugo-theme-ipng/layouts/partials/head.html

@@ -3,7 +3,7 @@
     <title>{{ .Site.Title }} {{ with .Title }}- {{ . }} {{ end }}</title>
     <link rel="stylesheet" type="text/css" href="{{ "css/fonts.css" | relURL }}">
     <link rel="stylesheet" type="text/css" href="{{ "css/fontawesome.css" | relURL }}">
-    {{ $options := dict "transpiler" "libsass" "targetPath" "css/styles.css" -}}
+    {{ $options := dict "transpiler" "dartsass" "targetPath" "css/styles.css" -}}
     {{ $style := resources.Get "styles.scss" | toCSS $options | minify | fingerprint -}}
     <link rel="stylesheet" type="text/css" href="{{ $style.RelPermalink }}">
     {{ with resources.Get "css/userstyles.css" }}