Ajout de GET /system/mailboxes_info/domain

src/business/mailbox.py

src/routes/system/__init__.py

src/routes/system/get_mailboxes_info.py

src/routes/test_mailbox.py

src/routes/test_mailbox_info.py

src/web_models/mailbox.py

tools/client-nushell/client.nu

tools/client-nushell/test/tests.nu