feat(webmail): refresh the inbox when new mail arrives

Feature request originally raised by @farrasrayhand in #429, separated here so the fixed email-deletion bug can be closed without losing this request.

The inbox currently requires a manual page reload or navigation to show newly received messages. It should refresh automatically when new mail arrives, or clearly offer newly available messages without reloading the page.

The original proposal was periodic polling (for example every 30–60 seconds) or an event-driven refresh. The implementation approach is still open.

This is not implemented in #892. That PR integrates the deletion fix from contributor PR #430; automatic inbox refresh remains separate feature work.