Add outline to :focus-visible state for input.filter__checkbox to ensure visibility in Safari.

Closes #3365103

In the current implementation, the :focus state styling is inherited from the Claro theme's elements.css file. The styling applies an outline and a box-shadow to indicate focus, using CSS custom properties for flexibility:

.page-wrapper *:focus {
  outline: var(--focus-outline); /* The outline color is transparent */
  box-shadow: var(--focus-box-shadow); /* Intended for visual focus indication and overrides the outline */
}

There is a notable inconsistency in how Safari renders box-shadow on focused elements, resulting in insufficient visual feedback for focus states. To address this and ensure uniform, accessible focus indicators across all browsers—including Safari, I implemented an additional style for the input.filter__checkbox element. This introduces an outline as a reliable fallback focus indicator.

merged