Load mails in pages for large mailboxes #4005

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

Part of #4000

User Story

As a user with large mailboxes, I want the mail list to load quickly and to load more mails while I scroll so that big mailboxes stay usable.

User Value

Today every mailbox is loaded completely in one request, and all mails are parsed and rendered at once. That gets slow and memory-heavy for real mailboxes.

Acceptance Criteria

• The first page (e.g. 50 mails) is loaded when a mailbox is opened, more pages load when scrolling to the end of the list
• A loading indicator is shown at the end of the list, and no more requests are sent after the last page
• Opening a mail by its id from the URL works even if it isn't on the first page
• Switching mailboxes cancels running page requests
• Unit tests cover paging and the end of the list

Backend: blocked by the paging item in the epic (no next and no position on /mailboxes/{id}/emails). Alternatively /emails?mailbox=&position=&limit= could be used (to be verified with real data).