Delete mails #4002

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 mail user, I want to delete mails so that my mailboxes stay clean.

User Value

Without delete, unwanted mail piles up and users have to use another client.

Acceptance Criteria

• A "Delete" action is available in the action bar
• Outside Trash, the mail is moved to the mailbox with role trash (PATCH /emails/{id} with mailboxIds)
• In Trash, the mail is deleted permanently (DELETE /emails/{id}) after a confirmation
• The mail is removed from the list, the next mail or the empty state is shown, and the counters update
• Errors show a message, and the list stays consistent
• Unit tests cover both paths

Backend: DELETE destroys permanently, moving to trash via PATCH still has to be verified (see epic).