Implement basic login function; add basic styling

src/Frontend/Dashboard/DashboardController.php

@@ -0,0 +1,28 @@
+<?php
+declare(strict_types=1);
+
+namespace MailAccountAdmin\Frontend\Dashboard;
+
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Slim\Views\Twig;
+
+class DashboardController
+{
+    /** @var Twig */
+    private $view;
+
+    public function __construct(Twig $view)
+    {
+        $this->view = $view;
+    }
+
+    public function showDashboard(Request $request, Response $response): Response
+    {
+        $renderData = [
+            'username' => $_SESSION['username'],
+        ];
+
+        return $this->view->render($response, 'dashboard.html.twig', $renderData);
+    }
+}