fix: search bar icon/placeholder overlap caused by specificity conflict

The base input rule uses a high-specificity :not() chain (0,8,1) that
overpowered .header-search-input-wrap input (0,1,1). The base padding
won, so the search input's padding-left (which makes room for the icon)
was overridden. Switched to #site-search-input (1,0,0) to win the
specificity battle.

app/public/assets/css/content-page.css

@@ -138,6 +138,11 @@
 	max-width: 100%;
 }
 
+#apropos-contacts,
+#apropos-intro {
+	border-bottom: none;
+}
+
 /* Section separators (about page only) */
 .page-content > article > section {
 	padding-bottom: var(--space-xl);