Align Existing Forms with Design System Form Guidelines

## Description

We recently added a new Forms page to the design system that documents our form design and UX best practices:

https://graylog2.github.io/design-system/?path=/docs/patterns-forms–docs

This guidance should serve as the reference for all new forms going forward. We should also gradually update existing forms to align with these standards and provide a more consistent user experience across the product.

Some improvements will require significant effort, but several core aspects should be relatively straightforward to standardize:

- [ ] Layout consistency. For example, consistently displaying form fields directly below their labels.
- [ ] Validation behavior. Standardizing when and how validation messages are shown.
- [ ] Required vs. optional fields. Using a consistent approach to communicate whether a field is required or optional.

This issue focuses on identifying and implementing these foundational improvements across existing forms.