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/module.php

@@ -21,6 +21,7 @@ return [
         ['path' => 'helpdesk/settings', 'target' => 'helpdesk/settings'],
         ['path' => 'helpdesk/settings/test-connection-data', 'target' => 'helpdesk/settings/test-connection-data'],
         ['path' => 'helpdesk/settings/diagnose-data', 'target' => 'helpdesk/settings/diagnose-data'],
+        ['path' => 'helpdesk/debitor-summary-data', 'target' => 'helpdesk/debitor-summary-data'],
         ['path' => 'helpdesk/debitor-contacts-data', 'target' => 'helpdesk/debitor-contacts-data'],
         ['path' => 'helpdesk/debitor-tickets-data', 'target' => 'helpdesk/debitor-tickets-data'],
         ['path' => 'helpdesk/debitor-ticket-categories-data', 'target' => 'helpdesk/debitor-ticket-categories-data'],