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/lib/Module/Helpdesk/Service/DebitorDetailService.php

@@ -104,6 +104,57 @@ class DebitorDetailService
         return ['ok' => true, 'tickets' => $tickets];
     }
 
+    /**
+     * Load a summary of ticket KPIs for a customer (exact counts, not sampled).
+     *
+     * @return array{ok: bool, total?: int, open?: int, last_activity?: string, error?: string}
+     */
+    public function loadTicketSummary(string $customerNo, string $customerName): array
+    {
+        $result = $this->loadTickets($customerNo, $customerName);
+
+        if (!$result['ok']) {
+            return $result;
+        }
+
+        return self::summarizeTickets($result['tickets'] ?? []);
+    }
+
+    /**
+     * Aggregate ticket KPIs from a raw ticket array.
+     *
+     * Used by both the service method and the data endpoint (which has its own cache layer).
+     *
+     * @param array<int, array<string, mixed>> $tickets
+     * @return array{ok: true, total: int, open: int, last_activity: string}
+     */
+    public static function summarizeTickets(array $tickets): array
+    {
+        $closedStates = ['Erledigt', 'Resolved', 'Closed', 'Geschlossen'];
+
+        $openCount = 0;
+        $lastActivity = '';
+
+        foreach ($tickets as $ticket) {
+            $state = (string) ($ticket['Ticket_State'] ?? '');
+            if (!in_array($state, $closedStates, true)) {
+                $openCount++;
+            }
+
+            $activityDate = (string) ($ticket['Last_Activity_Date'] ?? '');
+            if ($activityDate !== '' && $activityDate !== '0001-01-01T00:00:00Z' && $activityDate > $lastActivity) {
+                $lastActivity = $activityDate;
+            }
+        }
+
+        return [
+            'ok' => true,
+            'total' => count($tickets),
+            'open' => $openCount,
+            'last_activity' => $lastActivity,
+        ];
+    }
+
     /**
      * Load activity log for a ticket (for async data endpoint).
      *