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

@@ -0,0 +1,17 @@
+@*
+    The site's own small mark: a speech bubble (the board speaks for the parents)
+    holding a grown-up and a child. Inline SVG, so there is no extra request and
+    nothing from a third-party host. Decorative only -- the name next to it carries
+    the meaning, so it is hidden from screen readers. Colours come from tokens in
+    Signet.razor.css; the size is set by whoever places it (MainLayout.razor.css).
+*@
+<svg class="signet" viewBox="0 0 40 40" width="40" height="40" aria-hidden="true" focusable="false">
+    <path class="signet-bubble"
+          d="M8 4h24a6 6 0 0 1 6 6v15a6 6 0 0 1-6 6H18l-8 6v-6H8a6 6 0 0 1-6-6V10a6 6 0 0 1 6-6z" />
+    <g class="signet-figures">
+        <circle cx="15" cy="12" r="3.5" />
+        <path d="M9 26c0-5 2.7-8 6-8s6 3 6 8z" />
+        <circle cx="26" cy="16" r="2.6" />
+        <path d="M21.5 26c0-3.8 2-6 4.5-6s4.5 2.2 4.5 6z" />
+    </g>
+</svg>