Add site layout with header navigation and footer

Elternbeirat.Web/wwwroot/app.css

@@ -1,3 +1,23 @@
+/* Base styles. Self-hosted system font stack — no external fonts, for privacy. */
+html {
+    font-family: system-ui, -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
+    line-height: 1.6;
+    color: #222;
+}
+
+body {
+    margin: 0;
+}
+
+a {
+    color: #1f3a5f;
+}
+
+h1, h2, h3 {
+    line-height: 1.25;
+    color: #1f3a5f;
+}
+
 h1:focus {
     outline: none;
 }