Apply account credit on same-month rebook, surface credit on student page

A monthly/weekly lesson booked into a month whose billing date has
already passed is charged at booking time by LessonBooker::settle,
bypassing the daily scan where account credit is otherwise applied. So
rebooking a cancelled paid lesson within the same month charged the
family in full while their cancellation credit sat unused — billed twice
for the same slot.

Apply the payer's credit to that charge-at-booking payment for scheduled
offerings, mirroring the daily scan: a payment fully covered by credit
settles and confirms its lesson. Add PaymentService::findPayment so
settle can re-read the row after applyCredits writes to it.

Also surface the student's total account credit at the top of their
detail page when they hold a balance, so the studio sees it at a glance.

Co-authored-by: anthropic/claude-opus-4-8

templates/admin/student-detail.php

@@ -114,6 +114,39 @@ $renderLessons = static function (array $rows, bool $withActions = false): void
         <div class="notice notice-error is-dismissible"><p><?php echo esc_html($error); ?></p></div>
     <?php endif; ?>
 
+    <?php
+    /*
+     * Surface the credit balance up top the moment there is one, so the studio
+     * sees at a glance that this student is owed against future billing without
+     * scrolling to the Account credit section. Only shown when there is credit to
+     * report — a zero balance is not news. The full breakdown stays below.
+     */
+    ?>
+    <?php if ($canBilling && $creditBalance > 0) : ?>
+        <div class="notice notice-info inline">
+            <p>
+                <?php
+                printf(
+                    /* translators: %s: total available credit, e.g. "45.00 CAD" */
+                    esc_html__('Total account credit: %s', 'unsupervised-schedular'),
+                    '<strong>' . esc_html(number_format_i18n($creditBalance, 2) . ' ' . $creditCurrency) . '</strong>'
+                );
+                ?>
+                <?php if ($payer['id'] !== (int) $student->ID) : ?>
+                    <span class="description">
+                        <?php
+                        printf(
+                            /* translators: %s: name of the parent/guardian whose account holds the balance. */
+                            esc_html__('Held on %s’s account.', 'unsupervised-schedular'),
+                            esc_html($payer['name'])
+                        );
+                        ?>
+                    </span>
+                <?php endif; ?>
+            </p>
+        </div>
+    <?php endif; ?>
+
     <?php $detailUrl = static fn(int $id): string => add_query_arg(['page' => $pageSlug, 'student_id' => $id], admin_url('admin.php')); ?>
 
     <h2><?php esc_html_e('Account', 'unsupervised-schedular'); ?></h2>