feat: Mantine design system with dark mode

frontend/index.html

@@ -3,7 +3,26 @@
   <head>
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta name="theme-color" content="#1d4a79" />
+    <meta
+      name="description"
+      content="Kirche Einfach — KI-gestützte Verwaltungsplattform für Kirche"
+    />
     <title>Kirche Einfach</title>
+    <script>
+      try {
+        var s = localStorage.getItem("kirche-color-scheme");
+        if (
+          s === "dark" ||
+          (!s && matchMedia("(prefers-color-scheme: dark)").matches)
+        ) {
+          document.documentElement.setAttribute(
+            "data-mantine-color-scheme",
+            "dark",
+          );
+        }
+      } catch (e) {}
+    </script>
   </head>
   <body>
     <div id="root"></div>