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/assets/styles.scss

@@ -1,3 +1,4 @@
+@use "sass:color";
 // Colors
 $midnight-blue: #07608f;
 $midnight-matte: #4c7e99;
@@ -763,7 +764,7 @@ table {
   border: 2px solid $ipng-orange;
   tr {
     &:nth-child(even) {
-      background-color: darken($table-cell-background, 10%);
+      background-color: color.adjust($table-cell-background, $lightness: -10%);
     }
   }
   th, td {
@@ -771,7 +772,7 @@ table {
   }
   th {
     color: $ipng-orange;
-    background-color: lighten($table-header-background, 3%);
+    background-color: color.adjust($table-header-background, $lightness: 3%);
     border-bottom: 3px solid $ipng-orange;
     font-weight: bold;
   }