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/shortcodes/contact-box.html

@@ -1,5 +1,5 @@
 {{ with $.Page.Site.Params.social }}
-{{ $socialMap := $.Page.Site.Data.notrack.social }}
+{{ $socialMap := hugo.Data.notrack.social }}
 {{ $width := default "12em" (.Params.Get "width") }}
 {{ $height := default "auto" (.Params.Get "height") }}
 {{ $float := default "right" (.Params.Get "float") }}