Fix main menu link color in header and back-to-top functionality

Main menu links in the header now explicitly use
--mainmenu-nav-link-color via .container-header .nav-link
selector, preventing the generic --nav-link-color from bleeding
through when variables resolve to the body link color.

Back-to-top link:
- Switch icon from IcoMoon (icon-arrow-up) to Font Awesome
  (fa-solid fa-arrow-up) for consistency with the rest of the
  template.
- Add id="top" to the header element so the href="#top" fallback
  works even when JavaScript is disabled.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/media/css/template.css

@@ -14360,12 +14360,21 @@ li.current a {
 }
 
 .container-header .mod-menu>li>a,
-.container-header .mod-menu>li>span {
+.container-header .mod-menu>li>span,
+.container-header .nav-link,
+.container-header .mod-menu-main__link {
 	position: relative;
 	color: var(--mainmenu-nav-link-color, #fff);
 	text-decoration: none;
 }
 
+.container-header .nav-link:hover,
+.container-header .nav-link:focus,
+.container-header .mod-menu-main__link:hover,
+.container-header .mod-menu-main__link:focus {
+	color: var(--link-hover-color, #c3d6ff);
+}
+
 @media (min-width: 992px) {
 	.container-header .mod-menu>li::after {
 		position: absolute;