Alerts previously showed the raw Proxmox node name (e.g., "on pve") even
when users configured a display name (e.g., "SPACEX") via Settings or the
host agent --hostname flag. This affected the alert UI, email notifications,
and webhook payloads.
Add NodeDisplayName field to the alert chain: cache display names in the
alert Manager (populated by CheckNode/CheckHost on every poll), resolve
them at alert creation via preserveAlertState, refresh on metric updates,
and enrich at read time in GetActiveAlerts. Update models.Alert, the
syncAlertsToState conversion, email templates, Apprise body text, webhook
payloads, and all frontend rendering paths.
Related to #1188
