fix(helpdesk): show 'System' instead of 'Unknown user' for empty actors

Ticket log entries without Created_By (e.g. system-generated) now show
'System' instead of 'Unbekannter Benutzer'. Missing timestamps show
a dash instead of 'Unbekannte Zeit'.

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

modules/helpdesk/web/js/helpdesk-communication.js

@@ -31,7 +31,7 @@ function entryTimestamp(entry, t) {
   if (date && time) return `${date} ${time}`;
   if (entry.timestamp_iso) return entry.timestamp_iso;
   if (date) return date;
-  return t('Unknown time');
+  return '—';
 }
 
 function participantRole(entry) {
@@ -84,7 +84,7 @@ export function renderCommunicationFeed(entries, options = {}) {
 
   let html = '<ol class="helpdesk-comm-feed" role="list">';
   for (const entry of list) {
-    const actor = entry.actor || t('Unknown user');
+    const actor = entry.actor || 'System';
     const typeVariant = entry.type_variant || 'other';
     const type = entry.type || '';
     const stateSubtype = entry.state_subtype || '';