[feat] Delete a folder, Folder management/Retention

Add a Folder management menu (to view each folder size), with possibility to delete a folder (including possibility to delete Sent or Trash folder).

For each folder it could be also useful to set a 'retention period', at least for preconfigured folders like Inbox, Sent, Spam (Trash, Junk already have this in Alias settings, other folders could be included there separately).
Otherwise it could be sufficient to add a flag in Settings/Folders&labels menu, to enable retention and the days field for each folder.

No files, tests, or entry points are named. Start by locating the existing folder and Settings/Folders&labels interfaces, then determine how folder deletion and retention are currently handled for Sent, Trash, Spam, Inbox, and custom folders. Done should include an agreed scope and verified behavior for folder management, deletion, and configurable retention.

Written by the indexing model from the issue text.