settings UI: add Site Base URL + Alert Webhook URL/format fields

The webhook + site_base_url settings existed in the backend but the Email
Settings page had no inputs (hardcoded fields), and the settings composable's
reactive map didn't include the keys so they never loaded. Add the three inputs
(site base URL, webhook URL, webhook format select) and register the keys.

frontend/src/composables/systemSettings.js

@@ -108,6 +108,9 @@ export function useSystemSettings() {
     smtp_from_address: '',
     smtp_from_name: 'ShopDB',
     alert_recipients: '',
+    site_base_url: '',
+    alert_webhook_url: '',
+    alert_webhook_format: 'teams',
     // Audit
     audit_retention_days: 90,
     // Floor map blueprint (per-facility)