feat(admin): add Users management (list/create/reset/delete/toggle), seed test users; improve UI with modern styles

templates/layout.html

@@ -7,6 +7,7 @@
     <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootstrap.min.css" rel="stylesheet">
     <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap-icons@1.10.5/font/bootstrap-icons.css">
     <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;600&display=swap" rel="stylesheet">
+    <link rel="stylesheet" href="{{ url_for('static', filename='style.css') }}">
     <style>
         body {
             background-color: #f8f9fa;
@@ -50,6 +51,11 @@
         </button>
         <div class="collapse navbar-collapse justify-content-end" id="navbarNav">
             <ul class="navbar-nav">
+                {% if current_user.is_authenticated and current_user.is_admin %}
+                <li class="nav-item">
+                    <a class="nav-link{% if request.path.startswith('/admin/users') %} active{% endif %}" href="{{ url_for('manage_users') }}">Пользователи</a>
+                </li>
+                {% endif %}
                 <li class="nav-item">
                     <a class="nav-link{% if request.path.startswith('/quiz') %} active{% endif %}" href="{{ url_for('choose_survey') }}">Опросы</a>
                 </li>