feat(helpdesk): align module with core UI standards and fix KPI accuracy

- Add server-side ticket summary endpoint for exact KPI counts instead
  of client-side calculation from first 200 tickets
- Extract summarizeTickets() as shared static method to avoid duplication
- Replace custom .helpdesk-spinner CSS with core [aria-busy] pattern
- Migrate settings connection-test feedback to showAsyncFlash()
- Replace helpdesk-clickable-rows.js with delegated event handling
- Remove unused helpdesk-search.js (dead code, never loaded)
- Harmonize German wording: Debitoren → Kunden (8 i18n keys)
- Add 6 PHPUnit tests for loadTicketSummary()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/helpdesk/i18n/default_de.json

@@ -2,21 +2,21 @@
   "Helpdesk": "Helpdesk",
   "Helpdesk settings": "Helpdesk-Einstellungen",
   "Customers": "Kunden",
-  "Debtor search": "Debitorensuche",
-  "Search debtors": "Debitoren suchen",
-  "Search by name or debtor number...": "Suche nach Name oder Debitorennummer...",
+  "Debtor search": "Kundensuche",
+  "Search debtors": "Kunden suchen",
+  "Search by name or debtor number...": "Suche nach Name oder Kundennummer...",
   "Search": "Suchen",
-  "Please provide a customer number": "Bitte geben Sie eine Debitorennummer an",
+  "Please provide a customer number": "Bitte geben Sie eine Kundennummer an",
   "Minimum 2 characters": "Mindestens 2 Zeichen",
   "Please enter at least 2 characters.": "Bitte mindestens 2 Zeichen eingeben.",
-  "No debtors found": "Keine Debitoren gefunden",
+  "No debtors found": "Keine Kunden gefunden",
   "Try a different search term.": "Versuchen Sie einen anderen Suchbegriff.",
   "Could not connect to Business Central.": "Verbindung zu Business Central fehlgeschlagen.",
   "BC connection is not configured.": "BC-Verbindung ist nicht konfiguriert.",
   "BC connection is not configured. Please configure the connection in the settings.": "BC-Verbindung ist nicht konfiguriert. Bitte konfigurieren Sie die Verbindung in den Einstellungen.",
   "BC connection is not configured": "BC-Verbindung ist nicht konfiguriert",
   "Open settings": "Einstellungen öffnen",
-  "Debtor No.": "Debitoren-Nr.",
+  "Debtor No.": "Kunden-Nr.",
   "Name": "Name",
   "City": "Ort",
   "Phone": "Telefon",
@@ -24,18 +24,18 @@
   "Master data": "Stammdaten",
   "Contacts": "Ansprechpartner",
   "Tickets": "Tickets",
-  "Debtor details": "Debitoren-Details",
+  "Debtor details": "Kunden-Details",
   "Search name": "Suchname",
   "Address": "Adresse",
   "Postal code": "PLZ",
   "Salesperson": "Verkäufer",
   "No contacts found": "Keine Ansprechpartner gefunden",
-  "No contacts are linked to this debtor in Business Central.": "Diesem Debitor sind in Business Central keine Ansprechpartner zugeordnet.",
+  "No contacts are linked to this debtor in Business Central.": "Diesem Kunden sind in Business Central keine Ansprechpartner zugeordnet.",
   "No.": "Nr.",
   "Type": "Typ",
   "Job title": "Position",
   "No tickets found": "Keine Tickets gefunden",
-  "No tickets are linked to this debtor in Business Central.": "Diesem Debitor sind in Business Central keine Tickets zugeordnet.",
+  "No tickets are linked to this debtor in Business Central.": "Diesem Kunden sind in Business Central keine Tickets zugeordnet.",
   "Ticket No.": "Ticket-Nr.",
   "Description": "Beschreibung",
   "Status": "Status",
@@ -43,8 +43,8 @@
   "Contact": "Kontakt",
   "Support": "Bearbeiter",
   "Ticket": "Ticket",
-  "Debtor": "Debitor",
-  "Debtor not found": "Debitor nicht gefunden",
+  "Debtor": "Kunde",
+  "Debtor not found": "Kunde nicht gefunden",
   "Ticket not found": "Ticket nicht gefunden",
   "Back to search": "Zurück zur Suche",
   "Back": "Zurück",