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/web/css/helpdesk.css

@@ -15,29 +15,6 @@
     outline-offset: -2px;
   }
 
-  /* Async loading spinner */
-  .helpdesk-tab-loading {
-    display: flex;
-    align-items: center;
-    gap: 0.75rem;
-    padding: 2rem;
-    color: var(--app-muted, #6c757d);
-    font-size: 0.875rem;
-  }
-
-  .helpdesk-spinner {
-    width: 1.25rem;
-    height: 1.25rem;
-    border: 2px solid var(--app-border, #dee2e6);
-    border-top-color: var(--app-primary, #0d6efd);
-    border-radius: 50%;
-    animation: helpdesk-spin 0.6s linear infinite;
-  }
-
-  @keyframes helpdesk-spin {
-    to { transform: rotate(360deg); }
-  }
-
   /* Overview tab — header links */
   .app-stats-table-header .helpdesk-tab-link {
     float: right;