feat(mail): mailaccount write endpoints — add / update / delete

[chmmou]

Wires `add_mailaccount` / `update_mailaccount` / `delete_mailaccount` through the shared `kaswrite` seam as the next #13 write slice.

• `add` is reversible (not prompted); `update` and `delete` are gated by the feat(cli): destructive-write confirmation infrastructure (--yes / interactive prompt) #109 confirmation prompt. All three honour `--dry-run` (feat(cli): --dry-run for write commands #132) and emit a feat(cli): structured write-action audit log #131 audit record with the password redacted.
• `add` splits the address into `local_part`/`domain_part` and takes no `mail_login` (KAS generates it, echoed in `ReturnInfo`). Y/N/text toggles and XLIST folder names default to the KAS API defaults, so a bare `add --password ` is a complete create.
• Password key splits between actions: `--password` → `mail_password` on add, `mail_new_password` on update. `update` sends only the explicitly-changed flags; `--responder` is passed through verbatim.
• `delete_mailaccount` uses the louder "permanently delete" verb (drops the mailbox and all messages), shared with `delete_database`.
• Read fixtures refreshed to real responses; read mapping tests realigned (empty mailbox ⇒ `used_mailaccount_space` 0). Two add fault fixtures with mismatched fault codes were corrected by the maintainer; all fault fixtures are now pinned to their documented codes.

Closes #114