diag: error_log sync-event POSTs to trace missing toasts

Temporary diagnostic scaffolding — B-side logs every POST to the relay
(status + body snippet), A-side already console.debugs the GET result
via sync_debug. Lets us narrow down where the sender-side toast gets
lost.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/collab-buckets/backend.php

@@ -707,10 +707,14 @@ function collabSyncCards(PDO $db): array {
             // authored by someone else.
             if ($foreignAuthor && !empty($c['author_username'])) {
                 $cardTitle = trim((string)($fields['title'] ?? '')) ?: '(untitled)';
-                collabRelayCall('/api/sync-events', 'POST', [
+                $evRes = collabRelayCall('/api/sync-events', 'POST', [
                     'recipient' => $c['author_username'],
                     'body'      => $selfUser . ' synced: ' . $cardTitle,
                 ]);
+                @error_log('[hyprcrd] sync-event POST → ' . $c['author_username']
+                    . ' ok=' . ($evRes['ok'] ?? 'n/a')
+                    . ' status=' . ($evRes['status'] ?? '-')
+                    . ' err=' . substr((string)($evRes['error'] ?? ''), 0, 120));
             }
 
         }