KanbanBoard's board tabs implemented roving tabindex but never wired up
arrow-key navigation, making inactive tabs keyboard-unreachable. Adds
ArrowLeft/ArrowRight/Home/End handling that moves both selection and
focus between tabs, closing an unmet requirement from the original
design doc. Also adds aria-pressed to App's List/Board toggle buttons
so their active state is exposed to assistive technology, not just
conveyed visually via CSS class.
