admin nav: replace bare <a> links with <ul>/<li>, use aria-current instead of .active class

- templates/admin/head.php: all 7 nav links (+ conditional Modifier + Déconnexion)
  wrapped in <ul class="admin-nav__list">/<li>; .active class removed, replaced
  with aria-current="page" on each <a> based on $currentPage match
- Déconnexion link: removed inline style="margin-left:auto;opacity:.6;"; moved to
  new .admin-nav__logout <li> class in admin.css
- public/assets/admin.css: replaced .admin-nav__link rules with .admin-nav__list a
  selectors; added .admin-nav__list (flex list, gap 2.5rem, flex:1); added
  .admin-nav__list a[aria-current="page"] rule (border-bottom underline indicator);
  added .admin-nav__logout / .admin-nav__logout a for the push-right logout item
- Removes .admin-nav__link class entirely from the codebase (was only used in
  templates/admin/head.php and admin.css)

Fixes WCAG 2.4.6 (nav landmark content model), 1.4.1 (colour-only active indicator),
and section VIII of the semantic HTML admin audit.

public/assets/admin.css

@@ -47,7 +47,18 @@ html, body {
   font-weight: 400;
 }
 
-.admin-nav__link {
+/* Nav link list */
+.admin-nav__list {
+  list-style: none;
+  margin: 0;
+  padding: 0;
+  display: flex;
+  align-items: center;
+  gap: 2.5rem;
+  flex: 1;
+}
+
+.admin-nav__list a {
   font-size: 0.85rem;
   letter-spacing: 0.1em;
   text-transform: uppercase;
@@ -57,10 +68,28 @@ html, body {
   transition: opacity 0.15s;
 }
 
-.admin-nav__link:hover,
-.admin-nav__link.active {
+.admin-nav__list a:hover,
+.admin-nav__list a[aria-current="page"] {
+  opacity: 1;
+  color: #fff;
+}
+
+.admin-nav__list a[aria-current="page"] {
+  border-bottom: 1px solid rgba(255, 255, 255, 0.6);
+  padding-bottom: 1px;
+}
+
+/* Logout link pushed to the far right */
+.admin-nav__logout {
+  margin-left: auto;
+}
+
+.admin-nav__logout a {
+  opacity: 0.6;
+}
+
+.admin-nav__logout a:hover {
   opacity: 1;
-  color: var(--white);
 }
 
 /* Main content area */