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/sprint/task-list.tsx

@@ -136,7 +136,7 @@ function CreateTaskForm({ storyId, sprintId, onDone }: { storyId: string; sprint
       <div className="flex gap-2">
         <Input name="title" autoFocus placeholder="Taaknaam…" className="h-7 text-sm flex-1" required />
         <CreateSubmitButton />
-        <Button type="button" variant="ghost" size="sm" className="h-7" onClick={onDone}>×</Button>
+        <Button type="button" variant="ghost" size="sm" className="h-7" aria-label="Annuleer" onClick={onDone}>×</Button>
       </div>
       {state && 'error' in state && typeof state.error === 'string' && (
         <p className="text-xs text-destructive">{state.error}</p>