Enhance CSRF protection and user management features

- Introduce CSRF token generation and management improvements, ensuring tokens are consistently retrieved from cookies or request state.
- Update user creation logic to handle validation errors more gracefully, providing user feedback for invalid input and existing email addresses.
- Revise user management UI to display success and error messages, improving user experience during user creation.
- Refactor admin user access checks to include additional role validation.

app/templates/users.html

@@ -13,6 +13,12 @@
             <div class="card-header">
                 <h3>Neuen Benutzer anlegen</h3>
             </div>
+            {% if error_message %}
+            <p class="error">{{ error_message }}</p>
+            {% endif %}
+            {% if success_message %}
+            <p class="hint" style="color:var(--tk-success);font-weight:600;">{{ success_message }}</p>
+            {% endif %}
             <form method="post" action="/admin/users" class="form-grid">
                 <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
                 <div class="form-row">
@@ -25,7 +31,7 @@
                 </div>
                 <div class="form-row">
                     <label>Passwort
-                        <input type="password" name="password" required minlength="10">
+                        <input type="password" name="password" required minlength="10" placeholder="Mindestens 10 Zeichen">
                     </label>
                     <label>Rolle
                         <select name="role">