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/_default/rss.xml

@@ -9,14 +9,14 @@
         <language>{{.}}</language>
         {{end}}
 
-        {{ with .Site.Author.email }}
+        {{ with .Site.Params.social.email }}
         <managingEditor>{{.}}
-            {{ with $.Site.Author.name }} ({{.}}){{end}}
+            {{ with $.Site.Params.author }} ({{.}}){{end}}
         </managingEditor>
         {{end}}
 
-        {{ with .Site.Author.email }}
-        <webMaster>{{.}}{{ with $.Site.Author.name }} ({{.}}){{end}}
+        {{ with .Site.Params.social.email }}
+        <webMaster>{{.}}{{ with $.Site.Params.author }} ({{.}}){{end}}
         </webMaster>
         {{end}}
 
@@ -38,7 +38,7 @@
           <title>{{ .Title }}</title>
           <link>{{ .Permalink }}</link>
           <pubDate>{{ .Date.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
-          {{ with .Site.Author.email }}<author>{{.}}{{ with $.Site.Author.name }} ({{.}}){{end}}</author>{{end}}
+          {{ with .Site.Params.social.email }}<author>{{.}}{{ with $.Site.Params.author }} ({{.}}){{end}}</author>{{end}}
           <guid>{{ .Permalink }}</guid>
           <description>{{ .Summary | markdownify }}</description>
         </item>