Single frequency setting; per-user override; footer line

Admin-facing simplification:
- Dropped separate \"lookahead\" and \"historical lookahead\" tree
  prefs (and the once-per-month historical gate). A single
  \"send every N days\" number now drives both the cron cadence
  and the window each issue looks ahead for living + deceased
  events.
- Default 14, range 1–90, applies uniformly.

User-facing addition:
- The /my-account/{tree} subscription card gained an \"Email
  frequency\" select with options: use site default, weekly,
  every 2 weeks, monthly, every 2 months, quarterly. Stored as
  a per-tree-per-user preference.
- Dispatch now checks each recipient's own cadence against
  their own last-sent timestamp. Admin-added external addresses
  with no webtrees account always receive every run (no
  per-user state).
- Newsletter footer now reads \"You can change how often you
  receive this email, or unsubscribe entirely, in the Newsletter
  subscription section on your My account page\" — true now
  that the control exists.

German translations updated for the new strings; stale ones
removed.

resources/views/edit-account-page.phtml

@@ -181,6 +181,32 @@ use Fisharebest\Webtrees\Tree;
                 <div class="form-text">
                     <?= I18N::translate('You will receive a periodic email with upcoming birthdays and other family events from %s.', e($tree->title())) ?>
                 </div>
+
+                <?php
+                    $current_freq  = Configuration::userFrequencyDays($tree, $user);
+                    $tree_freq     = Configuration::frequencyDays($tree);
+                    $freq_labels   = [
+                        0  => I18N::translate('Use site default (every %d days)', $tree_freq),
+                        7  => I18N::translate('Weekly'),
+                        14 => I18N::translate('Every 2 weeks'),
+                        30 => I18N::translate('Monthly'),
+                        60 => I18N::translate('Every 2 months'),
+                        90 => I18N::translate('Quarterly'),
+                    ];
+                ?>
+                <div class="mt-3">
+                    <label for="newsletter_frequency" class="form-label">
+                        <?= I18N::translate('Email frequency') ?>
+                    </label>
+                    <select class="form-select" id="newsletter_frequency"
+                            name="newsletter_frequency" style="max-width: 22rem;">
+                        <?php foreach ($freq_labels as $days => $label) : ?>
+                            <option value="<?= $days ?>" <?= $days === $current_freq ? 'selected' : '' ?>>
+                                <?= e($label) ?>
+                            </option>
+                        <?php endforeach ?>
+                    </select>
+                </div>
             </div>
         </fieldset>
     <?php endif ?>