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.

resources/views/admin.phtml

@@ -167,6 +167,38 @@ use Illuminate\Support\Collection;
                     </div>
                 </div>
 
+                <fieldset class="row mb-3">
+                    <legend class="col-sm-3 col-form-label">
+                        <?= I18N::translate('Intro paragraph for the next email') ?>
+                    </legend>
+                    <div class="col-sm-9">
+                        <small class="form-text text-muted d-block mb-2">
+                            <?= I18N::translate('Shown once, above the upcoming events. Cleared automatically after the next successful send.') ?>
+                            <br>
+                            <?= I18N::translate('Formatted as Markdown — e.g. %1$s for emphasis, %2$s for a link.', '<code>**bold**</code>', '<code>[label](https://example.org)</code>') ?>
+                            <br>
+                            <?= I18N::translate('Personalisation tokens:') ?>
+                            <code>{{first_name}}</code>,
+                            <code>{{last_name}}</code>,
+                            <code>{{username}}</code>,
+                            <code>{{email}}</code>
+                        </small>
+                        <?php foreach (Configuration::supportedSubjectLocales() as $code => $label) : ?>
+                            <?php
+                                $field = 'intro-' . $id . '-' . $code;
+                                $val   = Configuration::introForLocale($module, $tree, $code);
+                            ?>
+                            <div class="mb-2">
+                                <label class="form-label small text-muted mb-1" for="<?= e($field) ?>">
+                                    <?= e($label) ?>
+                                </label>
+                                <textarea class="form-control" rows="6"
+                                          id="<?= e($field) ?>" name="<?= e($field) ?>"><?= e($val) ?></textarea>
+                            </div>
+                        <?php endforeach ?>
+                    </div>
+                </fieldset>
+
                 <div class="row mb-3">
                     <label class="col-sm-3 col-form-label">
                         <?= I18N::translate('Subscribed users') ?>