Refresh header and footer with signet, gradient, nav pills and three-column footer; fix box-sizing of design boxes

Elternbeirat.Web/Features/Layout/Signet.razor.css

@@ -0,0 +1,11 @@
+/* A white bubble with green figures. It sits on the header gradient and on the dark
+   footer, both of which always carry white text, so --color-on-header is right in
+   both places. The figures use the darker green so they stay distinct on the white
+   bubble (the lighter --color-accent would wash out). */
+.signet-bubble {
+    fill: var(--color-on-header);
+}
+
+.signet-figures {
+    fill: var(--color-accent-dark);
+}