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/js/helpdesk-settings.js

@@ -1,6 +1,7 @@
 /**
  * Helpdesk settings page — auth mode toggle and connection test.
  */
+import { showAsyncFlash } from '/js/components/app-async-flash.js';
 
 // Toggle OAuth2 vs Basic Auth field visibility
 const authModeSelect = document.getElementById('auth_mode');
@@ -23,13 +24,11 @@ if (authModeSelect) {
 
 // Connection test button
 const testButton = document.getElementById('helpdesk-test-connection-button');
-const testResult = document.getElementById('helpdesk-test-connection-result');
 
-if (testButton && testResult) {
+if (testButton) {
   testButton.addEventListener('click', async () => {
     testButton.disabled = true;
-    testResult.textContent = testButton.dataset.testingLabel || 'Testing...';
-    testResult.className = '';
+    testButton.setAttribute('aria-busy', 'true');
 
     try {
       const csrfMeta = document.querySelector('meta[name="csrf-token"]');
@@ -53,25 +52,19 @@ if (testButton && testResult) {
       const data = await response.json();
 
       if (data.ok) {
-        testResult.textContent = data.message || 'Connection successful';
-        testResult.style.color = 'var(--color-success, green)';
+        showAsyncFlash('success', data.message || 'Connection successful');
       } else {
         let msg = data.error || 'Connection failed';
-        if (data.debug?.url) {
-          msg += '\nURL: ' + data.debug.url;
-        }
         if (data.debug?.http_code) {
           msg += ' (HTTP ' + data.debug.http_code + ')';
         }
-        testResult.textContent = msg;
-        testResult.style.color = 'var(--color-error, red)';
-        testResult.style.whiteSpace = 'pre-wrap';
+        showAsyncFlash('error', msg);
       }
     } catch {
-      testResult.textContent = 'Connection failed';
-      testResult.style.color = 'var(--color-error, red)';
+      showAsyncFlash('error', 'Connection failed');
     } finally {
       testButton.disabled = false;
+      testButton.removeAttribute('aria-busy');
     }
   });
 }