fix(notifications): fix SMTP error surfacing, webhook button label, backup timestamp

- testSmtp now surfaces real nodemailer error instead of generic 'SMTP not configured' on send failure
- admin webhook test button uses correct i18n key (was showing 'Test-E-Mail senden' in all languages)
- backup created_at uses stat.mtime instead of unreliable stat.birthtime on Linux

client/src/pages/AdminPage.tsx

@@ -1353,7 +1353,7 @@ export default function AdminPage(): React.ReactElement {
                       disabled={!smtpValues.admin_webhook_url?.trim()}
                       className="px-4 py-2 border border-slate-300 text-slate-700 rounded-lg text-sm font-medium hover:bg-slate-50 transition-colors disabled:opacity-40"
                     >
-                      {t('admin.smtp.testButton')}
+                      {t('admin.notifications.testWebhook')}
                     </button>
                   </div>
                 </div>

server/src/services/backupService.ts

@@ -117,7 +117,7 @@ export function listBackups(): BackupInfo[] {
         filename,
         size: stat.size,
         sizeText: formatSize(stat.size),
-        created_at: stat.birthtime.toISOString(),
+        created_at: stat.mtime.toISOString(),
       };
     })
     .sort((a, b) => new Date(b.created_at).getTime() - new Date(a.created_at).getTime());

server/src/services/notifications.ts

@@ -399,9 +399,24 @@ export async function sendWebhook(url: string, payload: { event: string; title:
 }
 
 export async function testSmtp(to: string): Promise<{ success: boolean; error?: string }> {
+  if (!getSmtpConfig()) return { success: false, error: 'SMTP not configured' };
   try {
-    const sent = await sendEmail(to, 'Test Notification', 'This is a test email from TREK. If you received this, your SMTP configuration is working correctly.');
-    return sent ? { success: true } : { success: false, error: 'SMTP not configured' };
+    const config = getSmtpConfig()!;
+    const skipTls = process.env.SMTP_SKIP_TLS_VERIFY === 'true' || getAppSetting('smtp_skip_tls_verify') === 'true';
+    const transporter = nodemailer.createTransport({
+      host: config.host,
+      port: config.port,
+      secure: config.secure,
+      auth: config.user ? { user: config.user, pass: config.pass } : undefined,
+      ...(skipTls ? { tls: { rejectUnauthorized: false } } : {}),
+    });
+    await transporter.sendMail({
+      from: config.from,
+      to,
+      subject: 'TREK — Test Notification',
+      text: 'This is a test email from TREK. If you received this, your SMTP configuration is working correctly.',
+    });
+    return { success: true };
   } catch (err) {
     return { success: false, error: err instanceof Error ? err.message : 'Unknown error' };
   }