Cache first page of all mailboxes in the frontend

[st3iny]

Is your feature request related to a problem? Please describe.

Opening another mailbox for the first time takes some time because the envelope list has to be loaded. It would be nice to pre-fetch and cache the first page of all mailboxes in the background while the app is opened.

This could be restricted to the mailboxes which are configured to sync in the background to reduce the performance impact.

Good in combination with: #10715

Describe the solution you'd like

The Mail app's frontend should load the first page of all mailboxes in the background so that switching mailboxes feels snappier.

Describe alternatives you've considered

No response

Additional context

No response