fix navigation links, footer categories, product card structure, and social icons

- add missing cta_href to hero section and error page CTAs
- replace hardcoded footer shop links with real product categories
- restructure product cards with stretched-link pattern so category
  badges link to their collection page
- unify social icons: footer and contact page share the same default
  links from a single source in content.ex
- add search implementation plan (docs/plans/search.md, deferred)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/theme-semantic.css

@@ -229,6 +229,14 @@
   outline-offset: 2px;
 }
 
+/* Stretched link: makes a parent container fully clickable via ::after overlay */
+.stretched-link::after {
+  content: "";
+  position: absolute;
+  inset: 0;
+  z-index: 0;
+}
+
 /* Nav link styling with active state indicator */
 .shop-nav a,
 .shop-nav span {