Work item header UI improvements

A number of items in the Tasks (Work Item) header can be visually improved to provide a more polished experience:

• Use the correct type icon for parent item (currently uses issues, for issues this should be issue-type-issue, or issue-type-incident for incidents)
• Prevent cropping of entire focus ring on parent button

• Hide title border on dark mode

Batching these here as it could make sense to tackle these together, but they're not interdependent.