Improvement: align focus ring color to UI-SPEC accent token

[hydraInsurgent]

TL;DR

Focus rings across the app use zinc-500 instead of the accent blue defined in UI-SPEC.md.

Current State

Checkboxes and buttons show a gray focus ring (zinc-500) on focus.

Desired State

Focus rings use the accent color (#2563EB) as specified in the project design system.