feat(ST-607): WCAG AA accessibility fixes

- PBI row: role="button", tabIndex, aria-selected, onKeyDown (Enter/Space), focus-visible ring
- PBI "+" button: aria-label with priority name
- Sprint header cancel button: aria-label="Annuleer bewerken"
- Task create cancel button: aria-label="Annuleer"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/backlog/pbi-list.tsx

@@ -89,9 +89,13 @@ function SortablePbiRow({
     <div
       ref={setNodeRef}
       style={style}
+      role="button"
+      tabIndex={0}
+      aria-selected={isSelected}
       onClick={onSelect}
+      onKeyDown={(e) => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); onSelect() } }}
       className={cn(
-        'group flex items-center justify-between px-4 py-2 cursor-pointer transition-colors hover:bg-surface-container',
+        'group flex items-center justify-between px-4 py-2 cursor-pointer transition-colors hover:bg-surface-container focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-inset',
         isSelected && 'bg-primary-container text-primary-container-foreground'
       )}
     >
@@ -294,6 +298,7 @@ export function PbiList({ productId, pbis, isDemo }: PbiListProps) {
                       <button
                         onClick={() => setDialogState({ mode: 'create', productId, defaultPriority: priority })}
                         className="text-xs text-muted-foreground hover:text-foreground"
+                        aria-label={`Nieuw PBI aanmaken (${PRIORITY_LABELS[priority]})`}
                       >
                         +
                       </button>