feat: create, edit, and delete mailing lists

createMailingList, updateMailingList, and deleteMailingList existed in the API
but were never called, so lists could only be read.

Editing needed a read first. InboxMailingListReference carries only
id/rid/name/owner, so a settings sheet seeded from it would have offered an
empty description and Public visibility, and saving would have blanked the real
description and quietly changed who can see the list. The sheet now reads the
current values and refuses to save until it has them.

Clearing a description sends an explicit null via updateValue rather than a nil
subscript assignment, which would drop the key and leave the old text in place.
permitMime and rejectMime are left untouched rather than sent empty, which would
wipe the list's mime filters.

Edit and delete are gated on ownership: the subscriptions query that builds this
view returns lists the user follows, which is not the same as lists they own.
Non-owners keep the unsubscribe action instead. Deleting destroys the archive
for everyone, so the confirmation says exactly that.

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