Add user management to admin dashboard and refine button specificity

- Implement admin user management interface (, , ) with async CRUD and password updating
- Add Users tab link to admin navigation across  and
- Use  in  to lower specificity for native button resets, allowing modifier classes like  to take precedence

admin/menu.php

@@ -165,6 +165,7 @@ $menuGrouped = get_full_menu(false);
       <div class="admin-nav-links">
         <a href="index.php">Announcements</a>
         <a href="menu.php" class="active">Menu</a>
+        <a href="users.php">Users</a>
       </div>
       <div class="admin-user">
         <span>Logged in as <strong><?= htmlspecialchars($user['username']) ?></strong></span>