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.
Inactive tabs' aria-controls pointed at an ID that only existed for the
active board, and the one existing tabpanel wrapped just the breadcrumb
text instead of the actual columns/cards. Render all four board panels
always in the DOM with v-show instead of v-if so every tab's
aria-controls resolves to a real, persistent panel wrapping its real
content, per the WAI-ARIA Tabs pattern.
