fix(seo): restructure head partial, fix Twitter social and SEO improvements (#749)

* fix(scss): remove top-level parent selector for Dart Sass compatibility

Dart Sass strictly disallows top-level parent selectors (`&`), which causes build failures on newer Hugo versions (e.g. 0.158.0+).

* feat(styles): refactor special styles for friends and search layouts

* fix(seo): restructure head partial, fix Twitter social and SEO improvements

- Split head template into dedicated partial hierarchy for better maintainability
- Add full Twitter Cards support with dynamic site and creator metadata
- Update social media link prefix from twitter.com to x.com and rename label to X
- Fix Twitter/X share button and Buffer via parameter handling
- Add Mastodon verification meta tags and rel="me" links
- Improve SEO structured data for both homepage and blog posts
- Add test site configuration and test avatar asset

---------

Co-authored-by: Cell <1024@lruihao.cn>

assets/scss/pages/specials/_base.scss

@@ -0,0 +1,6 @@
+.special {
+  .single-title,
+  .single-subtitle {
+    text-align: right;
+  }
+}

assets/scss/pages/specials/_friends.scss

@@ -1,7 +1,7 @@
 // Style of layout friends
 @use "core/mixins" as *;
 
-&.friends {
+.special.friends {
   .friend-links {
     margin-top: 1rem;
     display: flex;

assets/scss/pages/specials/_index.scss

@@ -1,11 +1,3 @@
-@use "sass:meta";
-
-.special {
-  .single-title,
-  .single-subtitle {
-    text-align: right;
-  }
-
-  @include meta.load-css('friends');
-  @include meta.load-css('search');
-}
+@use "base.scss";
+@use "friends.scss";
+@use "search.scss";

assets/scss/pages/specials/_search.scss

@@ -1,7 +1,7 @@
 // Style of layout search
 @use "core/mixins" as *;
 
-&.search {
+.special.search {
   // Google CSE
   .gcse-searchbox:empty,
   .gcse-searchresults:empty {