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>

tests/CmsTest.php

@@ -311,7 +311,7 @@ final class CmsTest extends TestCase
     public static function rejectedButtonIcons(): iterable
     {
         yield 'Symbol vor v25' => ['icon', 'phone', 24];
-        yield 'unbekanntes Symbol' => ['icon', 'whatsapp', 25];
+        yield 'unbekanntes Symbol' => ['icon', 'telegram', 25];
         yield 'Symbol ist kein Text' => ['icon', ['phone'], 25];
         yield 'Stellung vor v25' => ['iconPosition', 'start', 24];
         yield 'unbekannte Stellung' => ['iconPosition', 'middle', 25];