One-shot bilingual intro paragraph with markdown + author avatar

- Admin can set a per-locale intro paragraph for the next issue on
  the preferences page; cleared automatically after a successful
  send. Stored in module_setting (longText) so multi-paragraph
  notes fit.
- Intro is rendered via webtrees' CommonMark factory (same flavour
  as notes) with raw HTML escaped, supports {{first_name}},
  {{last_name}}, {{username}}, {{email}} substitution per recipient.
- Two-column intro layout: tree contact user's linked Individual
  becomes the editorial portrait on the left. Their avatar is
  added to the per-recipient embed set so the inline image always
  resolves rather than falling through to a tree-page login link.
- Masthead now shows the tree URL under the title.
- Avatar source dimensions bumped 96→192 px and JPEG quality 75→88
  so portraits stay crisp at retina display ratios.

src/Module.php

@@ -170,6 +170,12 @@ class Module extends AbstractModule implements ModuleCustomInterface, ModuleConf
                 'Prepended to the email subject line. Leave a field blank to fall back to the generic prefix below.'
                                                               => 'Wird der E-Mail-Betreffzeile vorangestellt. Ein leeres Feld greift auf das generische Präfix unten zurück.',
                 'Generic'                                     => 'Allgemein',
+                'Intro paragraph for the next email'          => 'Einleitungsabsatz für die nächste E-Mail',
+                'Shown once, above the upcoming events. Cleared automatically after the next successful send.'
+                                                              => 'Wird einmalig über den anstehenden Ereignissen angezeigt. Wird nach dem nächsten erfolgreichen Versand automatisch geleert.',
+                'Personalisation tokens:'                     => 'Personalisierungs-Platzhalter:',
+                'Formatted as Markdown — e.g. %1$s for emphasis, %2$s for a link.'
+                                                              => 'Formatiert als Markdown — z. B. %1$s für Hervorhebung, %2$s für einen Link.',
             ],
             'nl' => [
                 'Email Newsletter'                            => 'E-mailnieuwsbrief',
@@ -283,6 +289,11 @@ class Module extends AbstractModule implements ModuleCustomInterface, ModuleConf
                     ->string('subject-' . $id . '-' . $code, '');
 
                 Configuration::setSubjectPrefixForLocale($tree, $code, $locale_prefix);
+
+                $intro = Validator::parsedBody($request)
+                    ->string('intro-' . $id . '-' . $code, '');
+
+                Configuration::setIntroForLocale($this, $tree, $code, $intro);
             }
 
             // Per-user subscription toggles. A users-roster marker is