Close the open menu on a click beside it and add social icons

The open burger menu now closes when someone clicks elsewhere, without a
script: an invisible layer that belongs to the menu button covers the
page while the menu is open. Facebook, Instagram and WhatsApp join the
icon set, so menu buttons can carry them. The stroke attributes of every
icon move into the stylesheet, which saves about 4 KB per page.

Hüfner gets a right-aligned menu with the quote button, Facebook and
Instagram as menu buttons.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/assets/admin.css

@@ -1,3 +1,5 @@
+/* Der Strich aller Symbole aus icon.html.twig, wie auf der Website (site.css). */
+svg.block-icon{fill:none;stroke:currentColor;stroke-width:1.6;stroke-linecap:round;stroke-linejoin:round}
 /* Administration: shared foundation and the management screens.
    The editor has its own stylesheet (editor.css) that only the editor and the media page load. */