Make login button spinner more robust by using Bootstrap's data-bs-submit attribute

[MatrixNeoKozak]

The login.html and otp_login.html templates manually disable the submit button, add a spinner, and submit the form. This is fragile and duplicates code. Instead, use Bootstrap's data-bs-submit attribute (or a common pattern with disabled and inline spinner) to ensure consistent UX and reduce JavaScript duplication. However, since data-bs-submit is not standard, we propose a cleaner approach: remove the manual click handler that disables the button and submits, and instead rely on the form's native submit event. Add a single event listener on the form to disable the button and show spinner on submit, preventing double-clicks. This simplifies the templates and improves accessibility.