Fix checkbox indentation

[DinneK]

module-name: Fix Checkbox Spacing

Problem

• Nested checkbox lists were styled by the top-level .contains-task-list rule (padding-left: 0), so they rendered flush with their parent instead of indented.
• USWDS renders the external-link icon via mask-image (not just background-image) in some versions, so mask-image: none (plus the -webkit- prefix) is needed to fully suppress it on TODO links.

Solution

• Added .contains-task-list .contains-task-list override with padding-left: 1.75em so nested task lists indent without affecting top-level lists
• Added mask-image: none; alongside existing -webkit-mask-image: none; to fully suppress USWDS's external-link icon on TODO links across browsers

Result

• Nested checkboxes now render indented under their parent item, as intended
• External-link icon no longer appears on TODO links in browsers that use the standard mask-image property

Test Plan

• Test locally by:
• Running: git run dev and navigating to http://localhost:8080/

[sachin-panayil]

LGTM!