feat(quick-260907-let): Knopf "Verbindung testen" im Postfach-Formular

- testEmailConnection im API-Klienten, POST email-config/test
- EmailAlertConfigForm: Testknopf vor Speichern, Wartezustand, gruene/rote
  Rueckmeldung, Formularaenderung raeumt vorherige Rueckmeldung weg
- Beschreibungsblock der Komponente korrigiert (Knopf existiert jetzt)
- Vier neue Schluessel unter tenderRadar.emailAlerts in de/en
- vi.mock-Fabriken in EmailAlertConfigForm.test.tsx und my-sources.test.tsx
  um testEmailConnection erweitert, zwei neue Testfaelle

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FYZcd3SSmo14QTqWx2KKzU

apps/web/src/lib/tender-radar-api.ts

@@ -569,3 +569,27 @@ export async function saveEmailConfig(
   }
   return res.json();
 }
+
+/**
+ * Test the connection to this user's own portal-alert mailbox WITHOUT
+ * saving anything (Quick 260907-let, WINDOWS #16). Same payload shape as
+ * `saveEmailConfig` — when `password`/`username` are left out, the server
+ * falls back to this user's stored, decrypted credentials.
+ * POST /modules/tender-radar/email-config/test
+ */
+export async function testEmailConnection(
+  payload: Partial<EmailAlertConfig> & { password?: string },
+): Promise<{ success: boolean; message?: string }> {
+  const res = await fetch(`${API_URL}/modules/tender-radar/email-config/test`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    credentials: 'include',
+    body: JSON.stringify(payload),
+  });
+  if (!res.ok) {
+    throw new Error(
+      await extractErrorMessage(res, 'Failed to test email-alert connection'),
+    );
+  }
+  return res.json();
+}