Mailbox creation flow upgraded to modal with auto-login

We've upgraded the mailbox creation flow with a more streamlined modal interface. When you create a mailbox, the email input field is automatically focused and the password is auto-generated for convenience. If you need to create additional mailboxes, you can toggle a checkbox to keep the modal open and create another one without closing it. The "Open mailbox" button in the success notification now redirects you directly to the webmail app with auto-login enabled, so you can start using your mailbox immediately.